packages: Turn off MANSPLITPKG by default

Manpages are very small, and having a separate manpages package for
every base package is quite annoying for users. Turn this option
off by default.

Discussed with: bapt
Reviewed by: manu, emaste
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D52417

(cherry picked from commit a8a18eeb5e1711ad27c3ee20eb342b73d3badffa)