restore PROXY protocol support

support for PROXY was removed in 43e28e2fef. After discussion,
it's worth keeping around in case of client compatibility issues,
and until /rehash support is more mature.

irc/commands.go

@@ -207,6 +207,11 @@ var Commands = map[string]Command{
 		handler:   privmsgHandler,
 		minParams: 2,
 	},
+	"PROXY": {
+		handler:      proxyHandler,
+		usablePreReg: true,
+		minParams:    5,
+	},
 	"RENAME": {
 		handler:   renameHandler,
 		minParams: 2,