warnings before attempting a mount.
(longopts): Add no-executable option.
(mount_commands): Ditto.
(opts): Ditto.
(usage): Ditto. Indicate that system mount is now the default.
(main): Accommodate no-executable option. Make system mount the default.
* umount.cc (usage): Indicate that system umount is now the default.
(main): Make system umount the default.