Make many counts of tracks and channels unsigned...

... And in some places where a library uses signed types, assert that
the reported number is not negative.

What led me to this, is that there are many places where a size_t value for
an allocation is the product of a number of channels and some other number.

src/import/RawAudioGuess.h

@@ -25,4 +25,4 @@
    SF_FORMAT value
 */
 int RawAudioGuess(const wxString &in_fname,
-                  int *out_offset, int *out_channels);
+                  int *out_offset, unsigned *out_channels);