hurd: Fix race between calling RPC and handling a signal

	* sysdeps/mach/hurd/i386/intr-msg.h (INTR_MSG_TRAP): Make
	_hurd_intr_rpc_msg_about_to global point to start of controlled
	assembly snippet. Make it check canceled flag.
	* hurd/hurdsig.c (_hurdsig_abort_rpcs): Only mutate thread if it passed
	the _hurd_intr_rpc_msg_about_to point.
	* hurd/intr-msg.c (_hurd_intr_rpc_mach_msg): Remove comment on mutation
	issue, remove cancel flag check.