MINOR: quic: Clarifications about transport parameters value

MINOR: quic: Clarifications about transport parameters value

This is becoming difficult to distinguish the default values for
transport parameters which come with the RFC from our implementation
default values when not set by configuration (tunable parameters).
Add a comment to distinguish them.
Prefix these default values by QUIC_TP_DFLT_ to distinguish them from
QUIC_DFLT_* value even if there are not numerous.
Furthermore ->max_udp_payload_size must be first initialized to
QUIC_TP_DFLT_MAX_UDP_PAYLOAD_SIZE especially for received value.