unshare: in usage text, reshuffle options into somewhat related groups

unshare: in usage text, reshuffle options into somewhat related groups

First the mount-related options (because --mount was the first
namespace option earlier) plus two other options about folders;
then the options about UIDS and GIDS; then two options about
forking; then two miscellaneous options; and lastly the two
options about time, as --time was the last namespace option.

Signed-off-by: Benno Schulenberg <bensberg@telfort.nl>