cap_rights_limit(2): CAP_ACCEPT also permits accept4(2).
Change the closeallfds() routine to use closefrom() when it is
available (closefrom() was added to FreeBSD in 8.0-release).
The selection is made at compile-time, as I still compile a
FreeBSD-based version of lpr&friends on other platforms.

While testing I out that (at least on my system) lpd has been
closing 11095 fd's, when there are only 6 fd's open.  The old
code took 120 times more clocktime than calling closefrom().
(although that was still less than 2/1000-ths of a second!)

Reviewed by:	jilles
MFC after:	2 weeks