Re-add support for non-portable socket options

Re-add support for non-portable socket options

Now that we support tunnelling arbitrary protocols over HTTP as
well as "100 Continue" responses, TCP_NODELAY actually becomes
useful to us.  TCP_NODELAY is actually reasonably portable
nowadays; even.

While we're adding non-portable options, TCP_CORK/TCP_NOPUSH can
be enabled, too.  Unlike some other servers, these can't be
disabled explicitly/intelligently to force a flush, however.
However, these may still improve performance with "normal" HTTP
applications (Mongrel has always had TCP_CORK enabled in Linux).

While we're adding OS-specific features, we might as well
support TCP_DEFER_ACCEPT in Linux and FreeBSD the "httpready"
accept filter to prevent abuse.

These options can all be enabled on a per-listener basis.