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

So the enums and structs were placed into http-t.h and the functions
into http.h. This revealed that several files were dependeng on http.h
but not including it, as it was silently inherited via other files.
diff --git a/src/vars.c b/src/vars.c
index 227c94b..edcf1af 100644
--- a/src/vars.c
+++ b/src/vars.c
@@ -2,7 +2,7 @@
 
 #include <haproxy/api.h>
 #include <common/cfgparse.h>
-#include <common/http.h>
+#include <haproxy/http.h>
 #include <haproxy/list.h>
 
 #include <types/vars.h>