(find_mount_point): Use save_cwd/restore_cwd.

(find_mount_point): Use save_cwd/restore_cwd.
"save-cwd.h": New include.

(show_dev): Deal with null values for DISK and FSTYPE,
including interpreting special 0 errno return from get_fs_usage.
(show_point): Don't fail if we can't find a mount entry for POINT,
just call show_dev with 0 values and let it fail if necessary.
(main): Require a non-zero MOUNT_LIST only if showing all filesystems.
(find_mount_point): New function.
(xgetcwd): New declaration.

All from Miles.