Implement pthread_setname_np and pthread_getname_np

These should be compatible with Linux, pthreads-win32 (Linux mode) and Cygwin
implementations (but not OSX, BSD or pthreads-win32 (BSD mode)).

Based on MSDN code[1].

[1] https://msdn.microsoft.com/en-us/library/xcb2z8hs%28v=vs.71%29.aspx