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