freebsd-update: improve mandoc db generation

freebsd-update compares the dates on man pages with mandoc.db, and if
any newer pages are found it regenerates mandoc.db.

Previously, if mandoc.db did not already exist the check failed and
freebsd-update then failed to create one. Now, check that mandoc.db
exists before performing the check for newer pages.

Reported by: bdrewery (in D10482)
Reviewed by: gordon
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29575