Fix MX checks for dynamic updates

Fix MX checks for dynamic updates

The check_mx() function in lib/ns/update.c incorrectly tests whether the
DNS_RDATA_CHECKMX/DNS_RDATA_CHECKMXFAIL flags are set for each applied
MX record update as these flags are never set in code paths related to
dynamic updates; they can only be set when loading a zone from a master
file (DNS_ZONEOPT_CHECKMX -> DNS_MASTER_CHECKMX -> DNS_RDATA_CHECKMX).
This flaw allows MX records containing IP addresses to be added to a
zone even when "check-mx fail;" is used.

Ensure correct behavior by modifying the relevant tests in check_mx() so
that they use DNS_ZONEOPT_CHECKMX/DNS_ZONEOPT_CHECKMXFAIL instead.