Increase listen backlog to deal with request spikes

In order to deal with transient request spikes don't limit the
connection backlog to 10 instead use a default appropriate to
the OS.

For FreeBSD and Mac OS this is -1 (system default) for everything
else this is 511 which will get truncated back to the OS max.
Merge pull request #552 from stevenh/master

Increase listen backlog to deal with request spikes