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.

sslgte279 and ssl3: Move the check for _sslobj to inside the loop
instead of above it. That way if the socket is closed while
reading/writing (actually, while waiting for the read or write event)
then the ValueError will be raised on the next loop iteration (because
closing cancels the read/write events) instead of a TypeError.

This may or may not obviate the chances for an OSError: closed to be
raised.

Make _sslgte279.py preserve the create_connection function from
gevent.socket instead of from stdlib ssl.

Don't run 'test_openssl_version' when ssl is backed with LibreSSL

On LibreSSL OPENSSL_VERSION_INFO is always (2, 0, 0, 0, 0), so the test fails
for all LibreSSL versions after 2.0.