Fix tv_divide().

The old code contained an integer overflow which caused the mp3 audio
format handler to compute the chunk of large mp3 files incorrectly.

Moreover, the code was more complicated than necessary. So replace it
by a simple calculation which just transforms the given struct timeval
into its number of microseconds, does the division and transforms
the quotient back to a struct timeval.

Thanks to Gerrit Renker for pointing out the problem.