ext/flac/gstflacenc.*: Handle non-zero start timestamps correctly, mark header packets as

ext/flac/gstflacenc.*: Handle non-zero start timestamps correctly, mark header packets as

Original commit message from CVS:
* ext/flac/gstflacenc.c: (gst_flac_enc_write_callback),
(gst_flac_enc_check_discont), (gst_flac_enc_chain),
(gst_flac_enc_change_state):
* ext/flac/gstflacenc.h:
Handle non-zero start timestamps correctly, mark header packets as
IN_CAPS and print a warning and suggest using audiorate if stream
discontinuities are detected. When FLAC supports flushing the encoder
somehow this should be done for discontinuities instead.
Remove some unused variables from the instance struct.