music: Allow / separators in substitute filenames.

For substitute music files we want to be able to specify relative
paths in a platform-independent way using Unix path separators.
Replace Unix-style / separators in the path that was read with the
path separator for the native system.
opl: Add API to adjust tempo.

When the tempo is changed, the times on all active timers must be
adjusted to match the new timing values. Add an API to do this and
invoke it when a tempo change meta event is read.
opl: Change library to use us instead of ms.

Multi-track MIDI files are played back using separate callback chains
for each track, and this introduces the possibility of one track
becoming out of sync with the others. This was noticeable in WADs
that use multi-track MIDIs, such as Alien Vendetta. Increase the
timing resolution to microsecond precision to fix this.
Merge branch 'master' of github.com:chocolate-doom/chocolate-doom