Use __FBSDID instead of the license agreement to embed the $FreeBSD$ RCS keyword

Reminded by a comment made by markj w.r.t. using __FBSDID in .c files ala a
past Phabricator review.

MFC after:	2 months
Sponsored by:	Dell EMC Isilon

lib/libcam/tests/libcam_test.c

@@ -22,10 +22,11 @@
  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
- *
- * $FreeBSD$
  */
 
+#include <sys/cdefs.h>
+__FBSDID("$FreeBSD$");
+
 #include <errno.h>
 #include <fcntl.h>
 #include <stdio.h>

lib/libsbuf/tests/sbuf_core_test.c

@@ -22,10 +22,11 @@
  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
- *
- * $FreeBSD$
  */
 
+#include <sys/cdefs.h>
+__FBSDID("$FreeBSD$");
+
 #include <sys/param.h>
 #include <sys/sbuf.h>
 #include <errno.h>

lib/libsbuf/tests/sbuf_stdio_test.c

@@ -22,10 +22,11 @@
  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
- *
- * $FreeBSD$
  */
 
+#include <sys/cdefs.h>
+__FBSDID("$FreeBSD$");
+
 #include <sys/param.h>
 #include <sys/sbuf.h>
 #include <errno.h>

lib/libsbuf/tests/sbuf_string_test.c

@@ -22,10 +22,11 @@
  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
- *
- * $FreeBSD$
  */
 
+#include <sys/cdefs.h>
+__FBSDID("$FreeBSD$");
+
 #include <sys/param.h>
 #include <sys/sbuf.h>
 #include <errno.h>