UPSTREAM: selinux: rename the {is,set}_enforcing() functions

Rename is_enforcing() to enforcing_enabled() and
enforcing_set() to set_enforcing().

Signed-off-by: Paul Moore <paul@paul-moore.com>

(cherry picked from commit e5a5ca96a42ca7eee19cf8694377308771350950)
Change-Id: I6063be6809663000835461787d9e43df2f6fd853
Bug: 140252993
Signed-off-by: Jeff Vander Stoep <jeffv@google.com>