Don't allow file descriptors to go above 64k, when we're closing them in a fork(2).

Don't allow file descriptors to go above 64k, when we're closing them in a fork(2).

This saves time, when, even though the system allows the process limit to be
that high, the practical limit is much lower.  Also introduce an additional
optimization, in the form of using the CLOEXEC flag to close descriptors at
the right time.

(closes issue #17223)
 Reported by: dbackeberg
 Patches:
       20100423__issue17223.diff.txt uploaded by tilghman (license 14)
 Tested by: dbackeberg

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@260292 65c4cc65-6c06-0410-ace0-fbb531ad65f3