player, client API: refactor cplayer init, reduce client API differences

Move the command line parsing and some other things to the common init
routine shared between command line player and client API. This means
they're using almost exactly the same code now.

The main intended side effect is that the client API will load mpv.conf;
though still only if config loading is enabled.

(The cplayer still avoids creating an extra thread, passes a command
line, and prints an exit status to the terminal. It also has some
different defaults.)

DOCS/client-api-changes.rst

@@ -25,6 +25,10 @@ API changes
 
 ::
 
+ --- mpv 0.9.0 will be released ---
+ 1.15   - mpv_initialize() will now load config files. This requires setting
+          the "config" and "config-dir" options. In particular, it will load
+          mpv.conf.
  --- mpv 0.8.0 is released ---
  1.14   - add mpv_wait_async_requests()
         - the --msg-level option changes its native type from a flat string to