unshare: don't mark "|" and ":" as part of the placeholders

unshare: don't mark "|" and ":" as part of the placeholders

The "|" is a meta-character, and should thus be left unmarked, while
the ":" (in the argument of --map-users/--map-groups) is a literal,
and should therefore be marked in bold.

Also, add the new 'subids' literal value for the above two options.

And in the bargain tweak a comment.

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