Fix bind_sockets if OS does not support IPv6
Doc updates from Zhigang Wang
Merge pull request #823 from schlamar/bind-sockets-ipv6

Fix bind_sockets if OS does not support IPv6
Set CLOEXEC on subprocess pipe endpoints so they are not inherited by the child.

This is what the subprocess module does when it creates pipes, and is
necessary so that a close of the writing side of the stdin pipe will
be recognized by the child process.
Fix default IOLoop in Subprocess.

Exit callbacks would fail for Subprocesses that did not explicitly
set an IOLoop.