In PDelay mechanism, the Master has to reply to PDelay_Req from pclocks
with PDelay_respond and PDelay_follow_up. Add pack/issue functions
for this messages
Signed-off-by: C.Prados <c.prados@gsi.de>

[WARNING: this commit was changed by Alessandro, adding a missing
"pdelay_req_hdr" field in pp_instance, and exporting msg_copy_header
that had been turned to static -- this must be fixed]