Compile tracks as individual object files

In order to reduce the memory needs during compilation, this patch series
splits out the tracks into individual object files (`trk?.c`).

Secondly, a `struct netcat_track` is added to a new header `netcat.h`, combining
all track information at one place.

Last, but not least, I've applied the patch from #28 on top of this changeset as well
(to avoid merge conflicts on your side).

I tried to split out the changes into individual steps for easier review.
What do you think?