Merge branch 't/resample'

Merge branch 't/resample'

Was cooking for quite some time. The merge conflicted slightly,
but this was easy to fix.

However, even with the conflict resolved, the merged tree would not
compile because after the merge para_play depends on libsamplerate,
but this dependency was not encoded in configure.ac.

There was no way to fix this issue in either of the two branches
involved without rewriting history: The recently merged t/afh_receiver
branch (which introduced para_play) had no idea of libsamplerate
while t/resample has no idea of para_play.

This merge commit fixes both issues.

	0eb69b resample filter: Implementation.
	cad284 resample filter: Infrastructure.
	37e0df check_wav: Ask parent nodes before falling back to defaults.
	216399 Replace check_wav_task by write_task.
	1af65c Move wav detection code to a separate file.
	d5dc1c write: Make wav-related config options modular.
	c25d04 para_filter: Call proper ->free_config method on shutdown.

Conflicts:
	m4/gengetopt/makefile