DecoderAPI: add decoder_seek_where_frame()
decoder/audiofile: use integer seek times
decoder/flac: use integer seek times
decoder/mpcdec: use integer seek times
decoder/mpg123: use integer seek times
decoder/opus: remove redundant decoder_timestamp() call

After seeking, the MPD core automatically refreshes the timestamp, and
thus discards the value from decoder_timestamp().
decoder/opus: use integer seek times
decoder/sndfile: use integer seek times
decoder/vorbis: use integer seek times
decoder/wavpack: use integer seek times
decoder/wildmidi: use integer seek times