u-a: Use the generic PACKAGE instead of a literal "dpkg" for textdomain

This unties a bit more update-alternatives from dpkg, so that it can be
more easily reused by third parties.

utils/update-alternatives.c

@@ -2309,8 +2309,8 @@ main(int argc, char **argv)
 	int i = 0;
 
 	setlocale(LC_ALL, "");
-	bindtextdomain("dpkg", LOCALEDIR);
-	textdomain("dpkg");
+	bindtextdomain(PACKAGE, LOCALEDIR);
+	textdomain(PACKAGE);
 
 	admdir = admindir_init();