The OSS checks were a big mess and quite buggy. This reimplementes them using
a declarative approach and clearly distinguishing between the various OSS
implementations. The code should now almost be auto-documenting.
We currently support the following implementations of OSS:
* platform-specific (with `sys/soundcard.h`)
* SunAudio (default on NetBSD and useable on OpenBSD even if we have sndio
support there).
* 4Front (default on FreeBSD)
Since now each OSS check also checks for the appropriate soundcard header,
remove the old soundcard check.
Many thanks to @bugmen0t for in depth info about all the BSDs.
Check #380 and #359 for more info on this commit.
