Fix #3749:

When a full upgrade from 2.1.x to 2.2 is being done, after decompress
tarball with 2.2 files, /bin/sh is not able to run a script using syntax
'sh scriptname'. Because of that, /tmp/post_upgrade_command and
/etc/rc.reboot are not executed ending on an incomplete and broken
update.

Since we need to fix it only touching 2.2 files, the only way I found to
workaround the situation is to use pre_upgrade_command script to:

- Copy /bin/sh to /tmp/sh.old
- Change /etc/rc.firmware to use sh.old when calling
post_upgrade_command and rc.reboot
- Kill current rc.firmware process
- Start a new rc.firmware using same parameters

This issue doesn't affect nanobsd or 2.2 -> 2.2 updates

tmp/pre_upgrade_command

@@ -6,5 +6,25 @@
 PRIOR_VERSION=`uname -r | cut -d'.' -f1`
 echo $PRIOR_VERSION > /tmp/pre_upgrade_version
 
+# Hack to workaround ticket #3749
+if [ "${PRIOR_VERSION}" = "8" ] && grep -q 'sh /etc/rc.reboot' /etc/rc.firmware; then
+	PROC=$(ps axwww | grep '/etc/rc.firmware *pfSenseupgrade')
+	PID=''
+	IMG=''
+	if [ -n "${PROC}" ]; then
+		PID=$(echo "${PROC}" | awk '{print $1}')
+		IMG=$(echo "${PROC}" | sed 's,^.*pfSenseupgrade *,,')
+	fi
+
+	if [ -n "${PID}" -a -n "${IMG}" -a -f "${IMG}" ]; then
+		cp -fp /bin/sh /tmp/sh.old
+		kill ${PID} >/dev/null 2>&1
+		kill -9 ${PID} >/dev/null 2>&1
+		sed -i '' -e 's,sh /,/tmp/sh.old /,' /etc/rc.firmware
+		/etc/rc.firmware pfSenseupgrade "${IMG}"
+		exit
+	fi
+fi
+
 rm /boot/kernel/*