input: lock for accessing struct input_ctx

The previous code was locking only the input queue. That was too weak since it
didn't protect the input_ctx data structure. So remove the locking on the queue
and lock all the public functions that interact with the input_ctx.

The private functions and public functions that do not act on the input_ctx
(there are quite some functions doing mp_cmd manipulations for instance) are
not locked.

Some changes by wm4. Use a recursive mutex, and restructure some code to be
less annoying with locks, such as converting code to single return, or
making use of the recursive mutex.