MINOR: quic: Send PING frames when probing Initial packet number space

MINOR: quic: Send PING frames when probing Initial packet number space

In very very rare cases, it is possible the Initial packet number space
must be probed even if it there is no more in flight CRYPTO frames.
In such cases, a PING frame is sent into an Initial packet. As this
packet is ack-eliciting, it must be padded by the server. qc_do_build_pkt()
is modified to do so.

Take the opportunity of this patch to modify the trace for TX frames to
easily distinguished them from other frame relative traces.

Must be backported to 2.7.