bsdinstall: stop messing with file descriptors

Throughout the bsdinstall script fd 3 is used by f_dprintf (set through
$TERMINAL_STDOUT_PASSTHRU). By closing file descriptor 3 here, the
final f_dprintf "Installation Completed ... does not work anymore.

By putting the code into a subshell, file descriptors can be edited
without interference with the calling script.

Reviewed by:		emaste
Differential Revision:	https://reviews.freebsd.org/D35113
Sponsored by:		Beckhoff Automation GmbH & Co. KG
MFC after:		3 days

(cherry picked from commit 1f7746d)

usr.sbin/bsdinstall/scripts/script

@@ -115,12 +115,13 @@ fi
 bsdinstall mount
 
 # Fetch missing distribution files, if any
-exec 3>&1
-export BSDINSTALL_DISTDIR=$(`dirname $0`/fetchmissingdists 2>&1 1>&3)
-FETCH_RESULT=$?
-exec 3>&-
-
-[ $FETCH_RESULT -ne 0 ] && error "Could not fetch remote distributions"
+(
+	exec 3>&1
+	export BSDINSTALL_DISTDIR=$(`dirname $0`/fetchmissingdists 2>&1 1>&3)
+	FETCH_RESULT=$?
+	exec 3>&-
+	return $FETCH_RESULT
+) || error "Could not fetch remote distributions"
 
 # Unpack distributions
 bsdinstall checksum