Simplify xl_iff_90x() a bit and only check ac->ac_multicnt since ac_multicnt
is also bumped for multicast ranges.

ok mikeb@
add a missing __END_DECLS that vanished somehow
partially revert e1a7be08cf828e6212b5d6130d3b63919dbb3061

this one locking change was actually wrong, go back to locking the uao.
remove some shonky code under UVMLOCKDEBUG