src/setup.c: Check netmask more carefully.

[crybaby ~/src/adns/build]RES_CONF_TEXT="sortlist 172.29.199.1/24" RES_OPTIONS="adns_checkc:entex" adnshost -ta www-cache.distorted.org.uk
adnshost: ../src/check.c:83: checkc_global: Assertion `!(ads->sortlist[i].base.s_addr & ~ads->sortlist[i].mask.s_addr)' failed.
Aborted

Unfortunately, `ccf_sortlist' only checked that the free bits of the
network address are clear when given a general netmask, so if you give a
prefix length or leave it implicit then you hit a consistency check
failure later.

Check the mask later on, after all of the variants are handled.  There's
a little bit of variation in the message.

Signed-off-by: Mark Wooding <mdw@distorted.org.uk>