freebsd-update: regenerate man page database after update

These are currently not reproducible because they're built by the
makewhatis on the freebsd-update build host, not the one in the tree.
Regenerate after update, and later we can avoid including it in
freebsd-update data.

PR:		214545, 217389
Reviewed by:	delphij
MFC after:	1 month
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D10482

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

@@ -2943,6 +2943,17 @@ Kernel updates have been installed.  Please reboot and run
 			cap_mkdb ${BASEDIR}/etc/login.conf
 		fi
 
+		# Rebuild man page databases, if necessary.
+		for D in /usr/share/man /usr/share/openssl/man; do
+			if [ ! -d ${BASEDIR}/$D ]; then
+				continue
+			fi
+			if [ -z "$(find ${BASEDIR}/$D -type f -newer ${BASEDIR}/$D/mandoc.db)" ]; then
+				continue;
+			fi
+			makewhatis ${BASEDIR}/$D
+		done
+
 		# We've finished installing the world and deleting old files
 		# which are not shared libraries.
 		touch $1/worlddone