net: LookupMX does not consider IP addresses as valid records

[horkhe]

This is a followup to #46979. I found another case where https://golang.org/cl/322230 broke the original logic. Even though MX records MUST contain domain names, occasionally they contain IP addresses. And this change considers IP addresses as invalid records and returns an error making delivery to respective email service providers impossible. The number of such cases is minuscule in the great schema things, but still we at Mailgun handle a few thousand messages to such email providers per hour. So in the spirit of Robustness Principal I believe a change should be made to allow IP addresses in MX records.

[horkhe]

By the way we ran into similar issue: the LookupSRV function does not work well with Nomad/Consule that exposes instance ip:port via SRV records, but LookupSRV enforces domain names. So guys, maybe you should stop trying to "fix" the Internet and just make resolver methods return whatever DNS servers provide you with. Or at least provide us with a way to disable the annoying validation.

[cagedmantis]

@ianlancetaylor @neild