common/wrappers: add date and uname wrappers.

uname:
  overwrites the hostname in command uname with the string 'void'.
date:
  uses the timestamp read from SOURCE_DATE_EPOCH as date.

common/wrappers/date.sh

@@ -0,0 +1,6 @@
+#!/bin/sh
+
+if [ "$SOURCE_DATE_EPOCH" ]; then
+	post="--date @$SOURCE_DATE_EPOCH"
+fi
+exec /usr/bin/date $@ $post

common/wrappers/uname.sh

@@ -0,0 +1,5 @@
+#!/bin/sh
+
+uname=$(/usr/bin/uname $@)
+rv=$?
+echo "$uname" | sed "s/\(^\| \)$(/usr/bin/uname -n)\($\| \)/\1void\2/"