Remove the no-at variants of the kern_xx() syscall helpers.  E.g., we
have both kern_open() and kern_openat(); change the callers to use
kern_openat().

This removes one (sometimes two) levels of indirection and
consolidates arguments checks.

Reviewed by:	mckusick
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Fix check for vendor-specific peripheral qualifier.

Submitted by:	anton.rang@isilon.com
MFC after:	1 week