Add LOCALBASE/share/man to the valid path for manpages

Also compress manpages in this location.

As a followup of a discussion which occured in 2017:
https://lists.freebsd.org/pipermail/freebsd-arch/2017-March/018115.html
And following:
https://svnweb.freebsd.org/base?view=revision&revision=315053
and
https://svnweb.freebsd.org/base?view=revision&revision=315142

All the supported FreeBSD version now supports share/man in manpath for
LOCALBASE As a result the ports tree can now accept it for manpage, but
more over migrate to this new path. Resulting in more consistency now the
manpages in base and ports would be in the relative path (under share/)
and a reduced amount of patching needed to port something to FreeBSD

Note1: this has already be done for GNU info pages earlier
Note2: due to the fact that for end user no functionnality will change during
the migration of the manpages to the new location and to avoid massive rebuild
of packages, it has been decided to not bump portrevision when migrating.

Reviewed by: mat (portmgr)
Differential Revision: https://reviews.freebsd.org/D23166