Fix back-to-back runs of sys/netinet/fibs_test;slaac_on_nondefault_fib6

Fix back-to-back runs of sys/netinet/fibs_test;slaac_on_nondefault_fib6

This test was failing if run twice because rtadvd takes too long to die.
The rtadvd process from the first run was still running when the
second run created its interfaces.  The solution is to use SIGKILL during
the cleanup instead of SIGTERM so rtadvd will die faster.

While I'm here, randomize the addresses used for the test, which makes bugs
like this easier to spot, and fix the cleanup order to be the opposite of
the setup order

PR:		217871
MFC after:	18 days
X-MFC-With:	315458
Sponsored by:	Spectra Logic Corp