system/skeleton: remove password expiration from shadow

system/skeleton: remove password expiration from shadow

The fields in /etc/shadow were set as follows:

root::10933:0:99999:7:::

This sets the date of last password change to Jan 1, 2000, the minimum
password age to 0 days, the maximum password age to near-infinity, and a
warning period of 7 days. In practice, this means the password never
expires. So all of this is quite useless.

On the other hand, mkusers creates lines without all of these options.
It just sets ::::: which disables password expiration completely.

To make things consistent, do the same for the skeleton entries.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>