fix and simplify dns_rdataset_equal() and _equalx()

fix and simplify dns_rdataset_equal() and _equalx()

if both rdataslabs being compared have zero length, return true.

also, since these functions are only ever called on slabheaders
with sizeof(dns_slabheader_t) as the reserve length, we can
simplify the API: remove the reservelen argument, and pass the
slabs as type dns_slabheader_t * instead of unsigned char *.