mp_common: silence warning

int64_t is normally long long int. On 64 bit systems, it's sometimes
defined to long int, though. It has the same size and semantics as
long long int, but is still a different type.
Prefix MP_ to keycode defines

Do this to reduce conflicts with <linux/input.h>, which uses the same
names for some identifiers.

Warning: this changes the meaning of MP_KEY_DOWN:

KEY_DOWN is renamed to MP_KEY_DOWN (cursor down key)
MP_KEY_DOWN is renamed to MP_KEY_STATE_DOWN (modifier for key down
state)