tail(1): Implement tac(1) as the reverse version of cat(1)

tail(1): Implement tac(1) as the reverse version of cat(1)

The tac(1) is the reverse version of cat(1) and is implemented with
'tail -rq'.

Update cat.1, rev.1 and tail.1 man pages to mention the new tac(1).

The tac.1 man page is obtained from NetBSD.  The implementation is also
inspired by NetBSD but is different.