MFC r357560, r357707: login.conf(5) mail capability

r357560:
login.conf(5): split MAIL env var out into a "mail" capability

This allows it to be easily suppressed in, e.g., the "daemon" class where it
will not be properly expanded.

This is a part of D21481.

Submitted by: Andrew Gierth <andrew_tao173.riddles.org.uk>

r357707:
login.conf(5): update commented-out standard, too

MAIL now has a mail capability, instead, as of r357560.