flacparse: Don't parse the complete FLAC frames but only look for valid frame headers

flacparse: Don't parse the complete FLAC frames but only look for valid frame headers

Thanks to Felipe Contreras for the suggestion. This is partially
based on his patches and makes flacparse more than 3.5 times faster.

Looking for valid frame headers is unlikely to give false positives
because every frame header is at least 9 bytes long, contains a
14 bit sync code and a 8 bit checksum over the first 8 bytes.

Fixes bug #631200.