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/include/common/hpack-enc.h b/include/common/hpack-enc.h
index 7d73b35..ede8d28 100644
--- a/include/common/hpack-enc.h
+++ b/include/common/hpack-enc.h
@@ -31,7 +31,7 @@
 #include <string.h>
 #include <haproxy/api.h>
 #include <haproxy/buf-t.h>
-#include <common/http.h>
+#include <haproxy/http-t.h>
 #include <import/ist.h>
 
 int hpack_encode_header(struct buffer *out, const struct ist n,