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.
diff --git a/src/fcgi.c b/src/fcgi.c
index 8cfb6d0..455cc22 100644
--- a/src/fcgi.c
+++ b/src/fcgi.c
@@ -24,7 +24,7 @@
  * OTHER DEALINGS IN THE SOFTWARE.
  */
 
-#include <common/fcgi.h>
+#include <haproxy/fcgi.h>
 #include <haproxy/buf.h>
 #include <haproxy/istbuf.h>