Inline implementation of inet_net_pton taken from OpenBSD.

--- smtpd/to.c	2015-10-02 01:46:21.000000000 +0200
+++ smtpd/to.c	2015-10-02 09:53:55.349046955 +0200
@@ -63,6 +63,190 @@
 
 static int temp_inet_net_pton_ipv6(const char *, void *, size_t);
 
+#if !defined(__GLIBC__)
+/*	$OpenBSD: inet_net_pton.c,v 1.5 2005/08/06 20:30:03 espie Exp $	*/
+
+/*
+ * Copyright (c) 1996 by Internet Software Consortium.
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND INTERNET SOFTWARE CONSORTIUM DISCLAIMS
+ * ALL WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL INTERNET SOFTWARE
+ * CONSORTIUM BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL
+ * DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR
+ * PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS
+ * ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS
+ * SOFTWARE.
+ */
+
+#include <assert.h>
+
+static int	inet_net_pton_ipv4(const char *, u_char *, size_t);
+
+/*
+ * static int
+ * inet_net_pton(af, src, dst, size)
+ *	convert network number from presentation to network format.
+ *	accepts hex octets, hex strings, decimal octets, and /CIDR.
+ *	"size" is in bytes and describes "dst".
+ * return:
+ *	number of bits, either imputed classfully or specified with /CIDR,
+ *	or -1 if some failure occurred (check errno).  ENOENT means it was
+ *	not a valid network specification.
+ * author:
+ *	Paul Vixie (ISC), June 1996
+ */
+int
+inet_net_pton(int af, const char *src, void *dst, size_t size)
+{
+	switch (af) {
+	case AF_INET:
+		return (inet_net_pton_ipv4(src, dst, size));
+	default:
+		errno = EAFNOSUPPORT;
+		return (-1);
+	}
+}
+
+/*
+ * static int
+ * inet_net_pton_ipv4(src, dst, size)
+ *	convert IPv4 network number from presentation to network format.
+ *	accepts hex octets, hex strings, decimal octets, and /CIDR.
+ *	"size" is in bytes and describes "dst".
+ * return:
+ *	number of bits, either imputed classfully or specified with /CIDR,
+ *	or -1 if some failure occurred (check errno).  ENOENT means it was
+ *	not an IPv4 network specification.
+ * note:
+ *	network byte order assumed.  this means 192.5.5.240/28 has
+ *	0x11110000 in its fourth octet.
+ * author:
+ *	Paul Vixie (ISC), June 1996
+ */
+static int
+inet_net_pton_ipv4(const char *src, u_char *dst, size_t size)
+{
+	static const char
+		xdigits[] = "0123456789abcdef",
+		digits[] = "0123456789";
+	int n, ch, tmp, dirty, bits;
+	const u_char *odst = dst;
+
+	ch = *src++;
+	if (ch == '0' && (src[0] == 'x' || src[0] == 'X')
+	    && isascii(src[1]) && isxdigit(src[1])) {
+		/* Hexadecimal: Eat nybble string. */
+		if (size <= 0)
+			goto emsgsize;
+		*dst = 0, dirty = 0;
+		src++;	/* skip x or X. */
+		while ((ch = *src++) != '\0' &&
+		       isascii(ch) && isxdigit(ch)) {
+			if (isupper(ch))
+				ch = tolower(ch);
+			n = strchr(xdigits, ch) - xdigits;
+			assert(n >= 0 && n <= 15);
+			*dst |= n;
+			if (!dirty++)
+				*dst <<= 4;
+			else if (size-- > 0)
+				*++dst = 0, dirty = 0;
+			else
+				goto emsgsize;
+		}
+		if (dirty)
+			size--;
+	} else if (isascii(ch) && isdigit(ch)) {
+		/* Decimal: eat dotted digit string. */
+		for (;;) {
+			tmp = 0;
+			do {
+				n = strchr(digits, ch) - digits;
+				assert(n >= 0 && n <= 9);
+				tmp *= 10;
+				tmp += n;
+				if (tmp > 255)
+					goto enoent;
+			} while ((ch = *src++) != '\0' &&
+				 isascii(ch) && isdigit(ch));
+			if (size-- <= 0)
+				goto emsgsize;
+			*dst++ = (u_char) tmp;
+			if (ch == '\0' || ch == '/')
+				break;
+			if (ch != '.')
+				goto enoent;
+			ch = *src++;
+			if (!isascii(ch) || !isdigit(ch))
+				goto enoent;
+		}
+	} else
+		goto enoent;
+
+	bits = -1;
+	if (ch == '/' && isascii(src[0]) && isdigit(src[0]) && dst > odst) {
+		/* CIDR width specifier.  Nothing can follow it. */
+		ch = *src++;	/* Skip over the /. */
+		bits = 0;
+		do {
+			n = strchr(digits, ch) - digits;
+			assert(n >= 0 && n <= 9);
+			bits *= 10;
+			bits += n;
+		} while ((ch = *src++) != '\0' &&
+			 isascii(ch) && isdigit(ch));
+		if (ch != '\0')
+			goto enoent;
+		if (bits > 32)
+			goto emsgsize;
+	}
+
+	/* Firey death and destruction unless we prefetched EOS. */
+	if (ch != '\0')
+		goto enoent;
+
+	/* If nothing was written to the destination, we found no address. */
+	if (dst == odst)
+		goto enoent;
+	/* If no CIDR spec was given, infer width from net class. */
+	if (bits == -1) {
+		if (*odst >= 240)	/* Class E */
+			bits = 32;
+		else if (*odst >= 224)	/* Class D */
+			bits = 4;
+		else if (*odst >= 192)	/* Class C */
+			bits = 24;
+		else if (*odst >= 128)	/* Class B */
+			bits = 16;
+		else			/* Class A */
+			bits = 8;
+		/* If imputed mask is narrower than specified octets, widen. */
+		if (bits < ((dst - odst) * 8))
+			bits = (dst - odst) * 8;
+	}
+	/* Extend network to cover the actual mask. */
+	while (bits > ((dst - odst) * 8)) {
+		if (size-- <= 0)
+			goto emsgsize;
+		*dst++ = '\0';
+	}
+	return (bits);
+
+ enoent:
+	errno = ENOENT;
+	return (-1);
+
+ emsgsize:
+	errno = EMSGSIZE;
+	return (-1);
+}
+#endif
+
 const char *
 sockaddr_to_text(struct sockaddr *sa)
 {