id: support specifying the user by user ID

id: support specifying the user by user ID

* src/id.c (usage): Remove 'name' from the synopsis,
implying that one can also specify by user ID.
(main): Like chown(1), call parse_user_spec() to implement
user name or ID lookup with appropriate precedence.
* doc/coreutils.texi (id invocation): Mention that
a user ID is supported and how '+' affects lookup order.
* tests/misc/id-groups.sh: Remove test now subsumed into...
* tests/misc/id-uid.sh: New test covering new interface.
* tests/local.mk: Rename the test.
* NEWS: Mention the new feature.
Addresses http://bugs.gnu.org/15421