net: LookupSRV doesn't validate record names are consistent

[rolandshoemaker]

LookupSRV returns the 'canonical name' alongside the slice of SRV records, which is typically used to determine whether the name that was passed to LookupSRV was CNAME'd to another name. The returned canonical name is extracted from the first returned record header NAME, but it is not verified that the rest of the records are consistent.

If a resolver returns a response containing records with various names, LookupSRV obscures this by returning just the first name. Ideally it should verify that all of the names are consistent, returning an error if there were multiple names returned.