MFC r314869,r314871,r314872:

r314869:

Alphabetically sort variables

The only content change is minor rewording around CLEANDIRS/CLEANFILES to
accomodate sorting order.

r314871:

Fix LINKS example in bsd.prog.mk

LINKS appends DESTDIR -- don't suggest double-append in example.

r314872:

Add bsd.man.mk references for MAN under bsd.lib.mk and bsd.prog.mk

The latter set of manpages directly consume bsd.man.mk, so the bsd.man.mk
behavior should be the source of truth for underlying behavior, whereas
the other manpage fragment descriptions should document how they tweak
the variable behavior, if at all (bsd.prog.mk does tweak the default
value, as noted in its description)