Do not print kernel debugging on console. In case of serial console
this can cause a really heavy load on system. Several kernel debugging
messages can be triggered even remotely (e.g. bad ARP replies).

Use kern.warning instead, so that really significant messages still
will be printed on console.

Reviewed by: current@
MFC after: 1 week
Security: this change fixes a DoS condition, when default system

		console is serial, and box is flooded with bogus ARP
		packets