Bracket struct mfc and struct rtdetq with #ifdef _KERNEL.
Match the bracketing in netstat.
Since the cleanup of MROUTING, ports have broken because they
expect to include <netinet/ip_mroute.h> without including
<sys/queue.h>. Fix breakage at source.

The real fix, of course, is to fix the MROUTING APIs by blowing them
away and replacing them with something else...