MINOR: quic: refactor frame encoding

MINOR: quic: refactor frame encoding

This patch is a direct follow-up of the previous one. This time,
refactoring is performed on qc_build_frm() which is used for frame
encoding.

Function prototype has changed as now packet argument is removed. To be
able to check frame validity with a packet, one can use the new parent
function qc_build_frm_pkt() which relies on qc_build_frm().

As with the previous patch, there is no function change expected. The
objective is to facilitate a future QMux implementation.