Export features as a module attributes

Export features as a module attributes

Currently it's hard to check from python if the current build of the
library has support for X or Y (e.g. acl from mode, etc.). So we export
this as module-level constants and this will allow users to check:
  if posix1e.HAS_ACL_FROM_MODE:
    acl = posix1e.ACL(mode=0644)