Minor a2play bugfixes

Added -v (version) option to usage message.

Now actually uses the requested number of output channels.
Updated k2intro to the current version from Kobo II

That is, the one in test/data. (The benchmark songs are not to be
updated beyond what's needed to make them play as intended.)
Added rtsubtest.c

Added a simple test, rtsubtest.c, that loads a song and plays two
instances of it on different realtime states. It sets up a state
with a substate, both realtime with actual audio drivers, so that
the states are asynchronously playing from the same physical song
and wave data.

The purpose of this test is to verify that substates actually work,
and that they can indeed piggyback on the data of their master
states as intended.