Make manylinux fixes

Allow resetting the C state for whether libev child handlers are active.

Use this when we disable them in gevent.signal so that next time we use
gevent.subprocess it will work.

Fixes #857.

Fixes #856

[skip ci]

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.

[skip ci]

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