slabtop: Check for bad d and o option combination

slabtop: Check for bad d and o option combination

If you run slabtop with the -d option and then -o option the
delay gets set to zero and it runs forever. slabtop now checks
for this combination and errors.

Adding a DEJAGNU test also found that none of the slabtop
checks were running so they got added to the list and only the
ones that need /proc/slabinfo (if not readable) are skipped.

References:
 #160