REORG: include: split common/htx.h into haproxy/htx{,-t}.h

Most of the file was a large set of HTX elements manipulation functions
and few types, so splitting them allowed to further reduce dependencies
and shrink the build time. Doing so revealed that a few files (h2.c,
mux_pt.c) needed haproxy/buf.h and were previously getting it through
htx.h. They were fixed.
29 files changed