pycompat: define operating system constants

As suggested by Ryan in D1019, it's cleaner if we use defined constants
instead of `osname == 'nt'` everywhere.

Differential Revision: https://phab.mercurial-scm.org/D1033

mercurial/pycompat.py

@@ -316,3 +316,7 b' else:'
     rawinput = raw_input
 
 isjython = sysplatform.startswith('java')
+
+isdarwin = sysplatform == 'darwin'
+isposix = osname == 'posix'
+iswindows = osname == 'nt'