Return from main instead of exiting

Right now it doesn't make a difference as the end of main
is currently never reached. However with some additional
tweaks it can be, and that would allow the application
destructor to be called for a nicer cleanup on exit.

gnucash/gnucash-cli.cpp

@@ -174,5 +174,5 @@ main(int argc, char **argv)
     application.parse_command_line (argc, argv);
     application.start (argc, argv);
 
-    exit(0);  /* never reached */
+    return 0;
 }

gnucash/gnucash.cpp

@@ -465,5 +465,6 @@ main(int argc, char ** argv)
 
     auto user_file_spec = t_file_spec {application.get_no_file (), application.get_file_to_load ()};
     scm_boot_guile (argc, argv, inner_main, &user_file_spec);
-    exit(0); /* never reached */
+
+    return 0;
 }