vifm: set mainProgram = vifm

Without the change `vifm-full` can't be ran as:

    $ nix run -f. vifm-full
    error: unable to execute '...-vifm-full-0.14/bin/vifm-full': No such file or directory

With the change both of the above run fine:

    $ nix run -f. vifm
    $ nix run -f. vifm-full

pkgs/by-name/vi/vifm/package.nix

@@ -69,6 +69,7 @@ stdenv.mkDerivation rec {
 
   meta = with lib; {
     description = "Vi-like file manager${lib.optionalString isFullPackage "; Includes support for optional features"}";
+    mainProgram = "vifm";
     maintainers = with maintainers; [ raskin ];
     platforms = if mediaSupport then platforms.linux else platforms.unix;
     license = licenses.gpl2;