Separate out error_t definition

so interfaces needing it can get it.

	* stdlib/errno.h (error_t): Move definition to...
	* bits/types/error_t.h: ... new header.
	* stdlib/Makefile (headers): Add bits/types/error_t.h.
	* sysdeps/mach/hurd/bits/errno.h (error_t): Move definition to...
	* sysdeps/mach/hurd/bits/types/error_t.h: ... new header.
	* sysdeps/mach/hurd/errnos.awk (error_t): Likewise.
	* hurd/hurd.h: Include <bits/types/error_t.h>
	* hurd/hurd/fd.h: Include <bits/types/error_t.h>
	* hurd/hurd/id.h: Include <errno.h> and <bits/types/error_t.h>
	* hurd/hurd/lookup.h: Include <errno.h> and <bits/types/error_t.h>
	* hurd/hurd/resource.h: Include <bits/types/error_t.h>
	* hurd/hurd/signal.h: Include <bits/types/error_t.h>
	* hurd/hurd/sigpreempt.h: Include <bits/types/error_t.h>