sane: add libcurl for escl backend, add runit service

Avahi support is now optional. The escl backend is only available
if Avahi is enabled.

When SANE is built with Avahi support, saned will spawn a child process
that is related to Avahi functionality. This process can't be stopped
by sv and will remain after the main saned process is terminated.
Reported upstream: https://gitlab.com/sane-project/backends/-/issues/499

srcpkgs/sane/files/saned/run

@@ -0,0 +1,3 @@
+#!/bin/sh
+[ -r ./conf ] && . ./conf
+exec saned -l -u _saned ${OPTS}

srcpkgs/sane/template

@@ -9,20 +9,26 @@ configure_args="--disable-locking --enable-ipv6 --enable-pthread
  --with-usb --docdir=/usr/share/doc/sane ac_cv_func_mmap_fixed_mapped=yes
  $(vopt_with snmp)"
 hostmakedepends="pkg-config python3"
-makedepends="avahi-libs-devel libjpeg-turbo-devel tiff-devel libgphoto2-devel
- v4l-utils-devel libusb-devel openssl-devel libxml2-devel
- $(vopt_if snmp net-snmp-devel)"
+makedepends="libjpeg-turbo-devel tiff-devel libgphoto2-devel v4l-utils-devel
+ libusb-devel openssl-devel libxml2-devel $(vopt_if snmp net-snmp-devel)
+ $(vopt_if avahi 'avahi-libs-devel libcurl-devel')"
 depends="$(vopt_if snmp net-snmp)"
 conf_files="/etc/sane.d/*.conf"
 short_desc="Scanner Access Now Easy"
 maintainer="Piraty <piraty1@inbox.ru>"
 license="custom:GPL-2.0-or-later-with-SANE-exception"
 homepage="http://www.sane-project.org/"
+changelog="https://gitlab.com/sane-project/backends/-/raw/master/NEWS"
 distfiles="https://gitlab.com/sane-project/backends/uploads/${_gitlab_release_hash}/sane-backends-${version}.tar.gz"
 checksum=3a28c237c0a72767086202379f6dc92dbb63ec08dfbab22312cba80e238bb114
 noshlibprovides="avoid false detection of device drivers"
 
-build_options="snmp"
+# additional group 'lp' is required by saned to access some all-in-one devices
+system_accounts="_saned"
+_saned_groups="lp,scanner"
+
+build_options="avahi snmp"
+build_options_default="avahi"
 
 # tests broken on 32bit.  https://gitlab.com/sane-project/backends/-/issues/157
 if [ "$XBPS_TARGET_WORDSIZE" = "32" ]; then
@@ -47,6 +53,8 @@ post_install() {
 	sed -n \
 		-e "/\/* sane - Scanner Access Now Easy./,/dynamic linking to load backends on demand./p" \
 		"${wrksrc}/backend/dll.c" > "${DESTDIR}/usr/share/licenses/sane/SANE-EXCEPTION"
+
+	vsv saned
 }
 
 libsane_package() {