scripting (lua/js): utils.getpid: make wrapper of pid property

We now have at least 3 scripting APIs which are trivial wrappers
around properties: mp.get_mouse_pos, utils.getcwd, utils.getpid.

After some discussion on IRC it was decided that it's easier for us to
maintain them as trivial wrappers than to deprecate them and inflict
pain on users and script authors, so currently no plan to deprecate.