Don’t include string.h from sys/un.h.

Don’t include string.h from sys/un.h.

sys/un.h needs strlen in order to define SUN_LEN, but does not need
any of the other things declared by string.h; the path of least
resistance is to prototype strlen locally.

Also, the construct being used to get the size of everything up to the
sun_path member contains a formal dereference of a null pointer and
therefore has undefined behavior.  Replace with __SOCKADDR_COMMON_SIZE.

Some old RPC code was relying on sys/un.h to provide all of string.h.

	* sys/un.h [__USE_MISC]: Don’t include string.h.
	Include bits/types/size_t.h.  Prototype strlen locally.
	Use __SOCKADDR_COMMON_SIZE for size of leading members of
	struct sockaddr_un.

	* nis/nis_domain_of.c, nis/yp_xdr.c, sunrpc/svc.c:
	Include string.h.

	* scripts/check-obsolete-constructs.py (HEADER_ALLOWED_INCLUDES):
	Update.