pkgbase: Add an src.conf option for splitting man pages

Man pages can be big in total, add an options to split man pages
in -man packages so we produce smaller packages.
This is useful for small jails or mfsroot produced of pkgbase.
The option is off by default.

Reviewed by: bapt, Mina Galić <me@igalic.co>
Differential Revision: https://reviews.freebsd.org/D29169
MFC after: 2 weeks

(cherry picked from commit c7e6cb9e08d6b51e677a9f5546b8e36d678687d0)