set close-on-exec flag on listen socket

Original patch from Jos Backus, modified to work on weird platforms
where Fcntl::FD_CLOEXEC may not be defined.

Jos Backus wrote:
> Mongrel, at least in 1.1.4, doesn't set the close-on-exec flag
> on the listen socket. This leads to trouble in programs that
> use Mongrel and fork other programs (like puppetmasterd, part
> of Puppet). For an example, see
> http://groups.google.com/group/puppet-dev/browse_thread/thread/463f0f8cfc80cc43

Files:
modified: /trunk/lib/mongrel.rb (try)