Support decoding directly to .aifc

[madah81pnz1]

Fixes so that flac doesn't default to WAVE for .aifc extension

[madah81pnz1]

I wanted to add a test for this in test_flac.sh, and it turns out we already have this for aiff:

convert_to_aiff ()
{
	run_flac "$2" $1.raw || die "ERROR converting $1.raw to AIFF"
	run_flac "$3" $1.flac -o $1.aiff || die "ERROR converting $1.raw to AIFF"
}

However, adding .aifc here in the same way would still pass even without this commit, since there is nothing that checks that the actual output file is AIFF or AIFF-C; the test would still pass but we get a WAV file with a .aifc file extension.

Maybe there is a portable way to use file to check what format the output file actually is, I can try to add it in next commit. File would return one of IFF data, AIFF audio, IFF data, AIFF-C compressed audio, RIFF (little-endian) data, WAVE audio, Microsoft PCM, 16 bit, stereo 44100 Hz, so grepping for "AIFF audio", "AIFF-C" or "WAVE audio" should do the trick.