go gen improvements (#524)

Remove the "gen" directory and move directives_generate.go out of it.
Add a build ignore tag so it isn't build by default. Cleanup the go gen
invocations so there are not seen as package docs.

Simplify the code a bit and don't run go gen twice.

coredns.go

@@ -1,5 +1,7 @@
 package main
 
+//go:generate go run directives_generate.go
+
 import "github.com/miekg/coredns/coremain"
 
 func main() {