Use __SCCSID for SCCS IDs in libkvm sources.

Rather than using #ifdef's around a static char array, use the
existing helper macro from <sys/cdefs.h> for SCCS IDs. To
preserve existing behavior, add -DNO__SCCSID to CFLAGS to not
include SCCS IDs in the built library by default.

Reviewed by: brooks, dab (older version)
Reviewed by: rgrimes
Differential Revision: https://reviews.freebsd.org/D15459