freebsd-update: Clarify unsupported release upgrade error message

Notify users that upgrading from -CURRENT or -STABLE is unsupported by
freebsd-update.

Also ensure --currently-running provides a correctly formatted release
(as done by -r).

PR:		234771
Submitted by:	Gerald Aryeetey <aryeeteygerald_rogers.com>
Reported by:	yuri
Reviewed by:	bcran
MFC after:	1 month
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D18803

usr.sbin/freebsd-update/freebsd-update.sh

@@ -304,6 +304,14 @@ config_TargetRelease () {
 	fi
 }
 
+# Pretend current release is FreeBSD $1
+config_SourceRelease () {
+	UNAME_r=$1
+	if echo ${UNAME_r} | grep -qE '^[0-9.]+$'; then
+		UNAME_r="${UNAME_r}-RELEASE"
+	fi
+}
+
 # Define what happens to output of utilities
 config_VerboseLevel () {
 	if [ -z ${VERBOSELEVEL} ]; then
@@ -442,7 +450,8 @@ parse_cmdline () {
 			NOTTYOK=1
 			;;
 		--currently-running)
-			shift; export UNAME_r="$1"
+			shift
+			config_SourceRelease $1 || usage
 			;;
 
 		# Configuration file equivalents
@@ -658,6 +667,18 @@ fetchupgrade_check_params () {
 	FETCHDIR=${RELNUM}/${ARCH}
 	PATCHDIR=${RELNUM}/${ARCH}/bp
 
+	# Disallow upgrade from a version that is not `-RELEASE`
+	if ! echo "${RELNUM}" | grep -qE -- "-RELEASE$"; then
+		echo -n "`basename $0`: "
+		cat  <<- EOF
+			Cannot upgrade from a version that is not a '-RELEASE' using `basename $0`. 
+			Instead, FreeBSD can be directly upgraded by source or upgraded to a 
+			RELEASE/RELENG version prior to running `basename $0`.
+		EOF
+		echo "System version: ${RELNUM}"
+		exit 1
+	fi
+
 	# Figure out what directory contains the running kernel
 	BOOTFILE=`sysctl -n kern.bootfile`
 	KERNELDIR=${BOOTFILE%/kernel}