add a new check for MX only (do not allow A record alone)

[rjbs]

An A record is enough to deliver mail. In practice, though, the lack of an MX record often means that mail will not be deliverable.

Add a stricter check that only allows MX, not A. See 0b7bfb2 and following for example code.

[tmorehouse]

I could separate out the A check into a separate sub (i.e. domaincheck), and then if an option is set for strict MX only the MX check routine would not perform the Hostname only Lookup.

Although this option should probably be added to the non Net::DNS option as well.