MINOR: quic: define first packet flag

MINOR: quic: define first packet flag

Received packets treatment has some difference regarding if this is the
first one or not of the encapsulating datagram. Previously, this was set
via a function argument. Simplify this by defining a new Rx packet flag
named QUIC_FL_RX_PACKET_DGRAM_FIRST.

This change does not have functional impact. It will simplify API when
qc_lstnr_pkt_rcv() is broken into several functions : their number of
arguments will be reduced thanks to this patch.

This should be backported up to 2.6.