tests: limit oobgc check to accepted sockets

Otherwise these tests fail if we start using IO#autoclose=true
on Ruby 1.9 (and also if we use IPv6 sockets for tests).
support SO_REUSEPORT on new listeners (:reuseport)

This allows users to start an independent instance of unicorn on
a the same port as a running unicorn (as long as both instances
use :reuseport).

ref: https://lwn.net/Articles/542629/
avoid IO_PURGATORY on Ruby 1.9+

Ruby 1.9 and later includes IO#autoclose=, so we can use it
and prevent some dead IO objects from hanging around.
http_server: fixup comments for PID file renaming

Thanks to Hongli Lai for noticing my typo.  While we're at it, finish up
a halfway-written comment for the EXDEV case