MINOR: quic: adjust Rx packet type parsing

MINOR: quic: adjust Rx packet type parsing

qc_parse_hd_form() is the function used to parse the first byte of a
packet and return its type and version. Its API has been simplified with
the following changes :
* extra out paremeters are removed (long_header and version). All infos
  are now stored directly in quic_rx_packet instance
* a new dummy version is declared in quic_versions array with a 0 number
  code. This can be used to match Version negotiation packets.
* a new default packet type is defined QUIC_PACKET_TYPE_UNKNOWN to be
  used as an initial value.

Also, the function has been exported to an include file. This will be
useful to be able to reuse on quic-sock to parse the first packet of a
datagram.

This should be backported up to 2.7.