Allow some options taking filenames to refer to mpv config dir

Add the mp_get_user_path() function, and make it expand special path
prefixes. Use it for some things in mpv which take filenames
(--input-config, --screenshot-template, opengl icc-profile suboption).

This allows accessing files in the mpv config dir without hardcoding the
config path by prefixing the path with ~~/. Details see manpage
additions.
manpage: mention that the "run" does not wait for the command