Add missing imports for posix.py for OS X

These don't seem to do harm on Linux.

mercurial/posix.py

@@ -9,7 +9,7 b' reference.'
 """
 
 from i18n import _
-import os, sys, osutil, errno, stat, getpass
+import os, sys, osutil, errno, stat, getpass, pwd, grp
 
 posixfile = file
 nulldev = '/dev/null'