Add MW_REST_API and MW_ENTRY_POINT

Add MW_REST_API and MW_ENTRY_POINT

Define the global constant MW_REST_API in rest.php, by analogy with
MW_API. Also generalize this by adding MW_ENTRY_POINT, which contains
the entry script name, "cli" or "unknown". This allows tests such as

  if ( MW_ENTRY_POINT !== 'index' )

which is probably what is really intended by defined('MW_API') in many
cases.

Change-Id: I24099f4cdd170de17afd6e1bbad67c9b204071fc