bsd.man.mk: add MANGROUPS

[ivy]

Summary

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..MAN8 into man
groups.

[sjg]

share/mk/bsd.man.mk

219

This logic is premised on MAN containing things like cat.1 foo.8 etc, is that going to also be true for MANGROUPS in general?

[ivy]

share/mk/bsd.man.mk

219

yes, it should be. essentially where before you did this:

MAN= foo.1 bar.1

now you could do this:

MANGROUPS= FOO BAR
FOO=foo.1
BAR=bar.1

so ${${__group}} -> ${FOO} -> foo.1 should be semantically equivalent to ${MAN} -> foo.1.

[ivy]

fix ${PACKAGE} + ${MLINKS}

discovered during testing that this didn't work properly:

PACKAGE=kernel-man
MLINKS=foo.1 bar.1

the links would be installed in the utilities-man package because the
manlinksinstall target wasn't using the group-specific TAG_ARGS.

now it works.

[sjg]

Looks reasonable