treewide: add meta.mainProgram to file-managers

pkgs/applications/file-managers/krusader/default.nix

···
     description = "Norton/Total Commander clone for KDE";
     license = licenses.gpl2Only;
     maintainers = with maintainers; [ sander turion ];
+    mainProgram = "krusader";
   };
 }

pkgs/applications/file-managers/mc/default.nix

···
     license = licenses.gpl2Plus;
     maintainers = with maintainers; [ sander ];
     platforms = with platforms; linux ++ darwin;
+    mainProgram = "mc";
   };
 }

pkgs/applications/file-managers/nnn/default.nix

···
     license = licenses.bsd2;
     platforms = platforms.all;
     maintainers = with maintainers; [ jfrankenau Br1ght0ne ];
+    mainProgram = "nnn";
   };
 })

pkgs/applications/file-managers/pcmanfm/default.nix

···
     description = "File manager with GTK interface";
     maintainers = [ maintainers.ttuegel ];
     platforms = platforms.linux;
+    mainProgram = "pcmanfm";
   };
 }

pkgs/applications/file-managers/ranger/default.nix

···
     license = licenses.gpl3Only;
     platforms = platforms.unix;
     maintainers = with maintainers; [ toonn magnetophon ];
+    mainProgram = "ranger";
   };
 }