freebsd-update: re-edit files if merge conflict markers remain

freebsd-update will open ${EDITOR} if conflicts occur while merging
updates to config files.  Inform the user if they've left conflict
markers behind, and go back to editing the file.

PR:		185546
PR:		229689
Reviewed by:	delphij
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D37703

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

@@ -2543,8 +2543,21 @@ The following file could not be merged automatically: ${F}
 Press Enter to edit this file in ${EDITOR} and resolve the conflicts
 manually...
 			EOF
-			read dummy </dev/tty
-			${EDITOR} `pwd`/merge/new/${F} < /dev/tty
+			while true; do
+				read dummy </dev/tty
+				${EDITOR} `pwd`/merge/new/${F} < /dev/tty
+
+				if ! egrep -q '^(<<<<<<<|=======|>>>>>>>)([[:space:]].*)?$' $(pwd)/merge/new/${F} ; then
+					break
+				fi
+				cat <<-EOF
+
+Merge conflict markers remain in: ${F}
+These must be resolved for the system to be functional.
+
+Press Enter to return to editing this file.
+				EOF
+			done
 		done < failed.merges
 		rm failed.merges