Add -p for port flag (#4653)

-p is the default flag in DNS software for setting the port, we use
'dns.port' because of preventing clashes with other caddy software
users. This is no longer an issue, so we can do what we want here.

Add -p to works like -dns.port.

(this PR includes generated manpage, but that shouldn't matter too much)

Signed-off-by: Miek Gieben <miek@miek.nl>

core/dnsserver/register.go

@@ -21,6 +21,7 @@ const serverType = "dns"
 // wise they potentially clash with other server types.
 func init() {
 	flag.StringVar(&Port, serverType+".port", DefaultPort, "Default port")
+	flag.StringVar(&Port, "p", DefaultPort, "Default port")
 
 	caddy.RegisterServerType(serverType, caddy.ServerType{
 		Directives: func() []string { return Directives },