Document that kill() is not always safe for greenlets that aren't generally exception safe. Link to article about same. Ref #846.

[skip ci]

[skip appveyor]

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.

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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.