input: use separate type for command definitions

Introduce a mp_cmd_def struct to define commands, instead of using
mp_cmd for this. This way each command parameter can be a m_option,
instead of m_option plus some more stuff.

Define the ARG_ macros directly in terms of the OPT_ macros. Not sure if
this makes it easier to read (maybe not, even if it looks simpler), but
at least it makes it easier to add other option types.

Another idea was adding a name for each parameter (so you could have
named parameters), but not today.
command: allow "current" as argument to playlist_remove command

Feature request from github issue #376.