Handle locked user type suffix ('u!')

[andy-bower]

From the sysusers.d manpage:

Type u may be suffixed with an exclamation mark ("u!") to create a fully locked account.

This patch adds such support to opensysusers.

1. split suffix from type.
2. add 'locked' argument to add_user invocation.
3. call usermod(8) in add_user to set account expiry to imply a locked account.

Note that useradd(8) ignores the -e option if a system account is being created so we need an extra command for this.