Rename Packable::Printf as Packable::appendf

It performs append semantics not replace semantics, and this also paves
the way for SBuf integration.

diff --cc src/HttpRequest.cc
Simple merge

diff --cc src/adaptation/icap/ModXact.cc
Simple merge

diff --cc src/adaptation/icap/OptXact.cc
Simple merge

diff --cc src/adaptation/icap/Xaction.cc
Simple merge

diff --cc src/client_side.cc
Simple merge

diff --cc src/errorpage.cc
Simple merge

diff --cc src/http.cc
Simple merge

diff --cc src/tunnel.cc
Simple merge