lua: make register_event() not overwrite previous event handler

Instead, chain them.

Note that there's no logic to prevent the other event handlers to be run
from an event handler (like it's popular in GUI toolkits), because I
think that's not very useful for this purpose.
lua: auto-load scripts from ~/.mpv/lua/

This is like passing them to --lua.
options: make --no-config block all auto-loaded configuration files

Until now, the --no-config was explicitly checked in multiple places to
suppress loading of config files.

Add such a check to the config path code itself, and refuse to resolve
_any_ configuration file locations if the option is set.

osc.lua needs a small fixup, because it didn't handle the situation when
no path was returned. There may some of such cases in the C code too,
but I didn't find any on a quick look.