Apply more mechanical simplifications using recent Go features

This is a follow-up on the `go fix` run in commit
0210c5fd92.

That commit recorded the automated updates.

This one applies further simplifications that are mechanical and do not
involve logic changes, but are outside of what `go fix` did.

It also removes some now-obsolete comments and statements.

dnsoverride.go

@@ -1,9 +1,7 @@
 // Support for overriding DNS lookups, for testing purposes.
 // This is only used in tests, when the "dnsoverride" tag is active.
-// It requires Go >= 1.8.
 //
 //go:build dnsoverride
-// +build dnsoverride
 
 package main