Use common licence attributes from lib/licenses.nix

Many (less easily automatically converted) old-style strings
remain.

Where there was any possible ambiguity about the exact version or
variant intended, nothing was changed. IANAL, nor a search robot.

Use `with stdenv.lib` wherever it makes sense.

pkgs/tools/networking/maildrop/default.nix

@@ -12,9 +12,9 @@ stdenv.mkDerivation rec {
 
   patches = [ ./maildrop.configure.hack.patch ]; # for building in chroot
 
-  meta = {
+  meta = with stdenv.lib; {
     homepage = http://www.courier-mta.org/maildrop/;
     description = "Mail filter/mail delivery agent that is used by the Courier Mail Server";
-    license = [ "GPLv3" ];
+    license = with licenses; gpl3;
   };
 }