Add --currently-running <release> option to freebsd-update.

This option tells freebsd-update to act as if it is running a specific
release instead of querying the kernel.  In particular, this can be
useful when upgrading jails.

Requested by:	EuroBSDCon devsummit jails session
Tested by:	allanjude
MFC after:	1 week

usr.sbin/freebsd-update/freebsd-update.8

@@ -113,6 +113,11 @@ Please do not run
 .Nm Cm fetch
 from crontab or similar using this flag, see:
 .Nm Cm cron
+.It Fl -currently-running Ar release
+Don't detect the currently-running release; instead, assume that the
+system is running the specified
+.Ar release .
+This is most likely to be useful when upgrading jails.
 .El
 .Sh COMMANDS
 The

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

@@ -52,6 +52,8 @@ Options:
                   (default: root)
   --not-running-from-cron
                -- Run without a tty, for use by automated tools
+  --currently-running release
+               -- Update as if currently running this release
 Commands:
   fetch        -- Fetch updates from server
   cron         -- Sleep rand(3600) seconds, fetch updates, and send an
@@ -434,6 +436,9 @@ parse_cmdline () {
 		--not-running-from-cron)
 			NOTTYOK=1
 			;;
+		--currently-running)
+			shift; export UNAME_r="$1"
+			;;
 
 		# Configuration file equivalents
 		-b)