REORG: include: move common/fcgi.h to haproxy/

The file was moved almost verbatim (only stdio.h was dropped as useless).
It was not split between types and functions because it's only included
from direct C code (fcgi.c and mux_fcgi.c) as well as fcgi_app.h, included
from the same ones, which should also be remerged as a single one.
4 files changed