cr_gid is already used by Darwin

cr_gid is already used by Darwin

Commit eb8e55bba7740a87e07ef07bb4b789e6d4e36f0d introduced a variety
of functions for accessing members of the credentials structure in a
platform independent way. Sadly, cr_gid is already defined by the
Darwin platform headers (on Darwin, the GID is just the first of
the user's groups)

Turn cr_gid() into afs_cr_gid() to avoid this problem, and for
consistency, also rename cr_uid, cr_ruid, cr_rgid, and the
corresponding set_* functions.

Change-Id: I82d74900324d079b5a5c8985510a2c9586c4a526
Reviewed-on: http://gerrit.openafs.org/805
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>