Travis and Appveyor are both green.

Unify lots of implementation details between FileObjectPosix/Thread, including making *bufsize* of 0 be respected.

Enable get_number_open_files on more platforms, and use that to track down the refcycle in FileObjectThread.

Also, support bufsize of 0 in FileObjectPosix.

Fixes #840

Fixes #825.

[skip ci]

Convert readthedocs link for their .org -> .io migration for hosted projects

As per [their blog post of the 27th April](https://blog.readthedocs.com/securing-subdomains/) ‘Securing subdomains’:

> Starting today, Read the Docs will start hosting projects from subdomains on the domain readthedocs.io, instead of on readthedocs.org. This change addresses some security concerns around site cookies while hosting user generated data on the same domain as our dashboard.

Test Plan: Manually visited all the links I’ve modified.

Instead, we only start checking the timer after the first attempted
send. This is what the C implementation in CPython does.

Fixes benoitc/gunicorn#1282.

This makes building work on platforms lacking /bin/sh.

Python 2 issue only.

Allow setting attributes on sys.modules['gevent.signal']; they go to the
gevent/signal.py module for symmetry with getattr and dir.

Reloading results in leaks so don't bother checking for them.