py3: add a bytes version of os.name

os.name returns unicodes on py3. Most of our checks are like
    os.name == 'nt'

Because of the transformer, on the right hand side we have b'nt'. The
condition will never satisfy even if os.name returns 'nt' as that will be an
unicode.
We either need to encode every occurence of os.name or have a
new variable which is much cleaner. Now we have pycompat.osname.
There are around 53 occurences of os.name in the codebase which needs to
be replaced by pycompat.osname to support Python 3.

mercurial/pycompat.py

@@ -10,6 +10,7 b' This contains aliases to hide python ver'
 
 from __future__ import absolute_import
 
+import os
 import sys
 
 ispy3 = (sys.version_info[0] >= 3)
@@ -34,9 +35,10 b' else:'
 if ispy3:
     import builtins
     import functools
-    import os
     fsencode = os.fsencode
     fsdecode = os.fsdecode
+    # A bytes version of os.name.
+    osname = os.name.encode('ascii')
 
     def sysstr(s):
         """Return a keyword str to be passed to Python functions such as
@@ -82,6 +84,8 b' else:'
     def fsdecode(filename):
         return filename
 
+    osname = os.name
+
 stringio = io.StringIO
 empty = _queue.Empty
 queue = _queue.Queue