enforce the "include <config.h> first" rule

enforce the "include <config.h> first" rule

* qemud/Makefile.am: Ensure that the generated remote_protocol.c
includes <config.h> first.
* Makefile.maint (sc_require_config_h_first): New rule, so that
"make syntax-check" enforces this.
* .x-sc_require_config_h_first: New file.
* Makefile.am (.x-sc_require_config_h_first): Add it.
* qemud/remote_protocol.x: Don't include <config.h> here.
* qemud/remote_protocol.c: Regenerate.