bsd.man.mk: add MANGROUPS

MANGROUPS allows manpages to be divided into groups with their own
options, analogous to CONFGROUPS. This is mainly used for installing
manpages into the correct package by setting ${<group>PACKAGE}.

Convert the backward-compatibility variables MAN1..MAN9 into man
groups.

Reviewed by: sjg
Differential Revision: https://reviews.freebsd.org/D51490