stream_radio: fix some things

Using the radio set/step channel commands would have crashed (that was
broken for about a year, nobody ever noticed). The "capture" part of
a radio:// URI was incorrectly passed (this was broken quite recently).

Still couldn't test it fully. I have no radio device. I suspect nobody
uses this feature or will ever use it again.
mp_msg: fix typo in comment
Merge branch 'master' into lua_experiment
load each script in its own lua context
put some helper functions in mp. namespace

Also, allow passing explicit input section names to these helper
functions. They are optional, and fall back to a generated section name
unique to the script.
handle builtin moduels differently

e.g. "assdraw" is now only loaded if it's actually needed.

Namespace all of these modules properly.

"mp" is still a magic non-module, although it can be "require"d now.
integrate osc in mpv binary

The OSC is now loaded with --lua="@osc". This will use the baked-in copy
of core/lua/osc.lua.

I'm not rerally sure whether "@osc" is a nice syntax, but whatever.
allow loading multiple lua scripts