Rewrote timer code.

The new code is based on timestamps, rather than delays. This eliminates the
need to update every timer in timer_process(), which cost a non-trivial amount
of CPU time on low-end machines.

This involved changes to every user of the timer code, so this change almost
certainly introduces bugs.

src/fdd.h

@@ -1,5 +1,5 @@
 #define SEEK_RECALIBRATE -999
-void fdd_seek(int drive, int track_diff);
+uint64_t fdd_seek(int drive, int track_diff);
 int fdd_track0(int drive);
 int fdd_getrpm(int drive);
 void fdd_set_densel(int densel);