Also install 2.7.12 on travis.

Also update the SSL test for Python 2.7 to test the fix for Python bug
 no. 23804. This may need some tweaking for the 2.7.8 environment to
disable some tests. We'll let Travis run 2.7.8 to find out.

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.