newusers: Allow creation without aging information

newusers: Allow creation without aging information

If PASS_MAX_DAYS is not set, newusers falls back to 10000 days, which is
considered "unlimited" in some parts of the source tree. All other tools
fall back to -1, which truely implies unlimited.

Sync newusers with all other shadow tools.

How to reproduce:

1. Remove or comment out PASS_MAX_DAYS from /etc/login.defs
2. Run `newusers <<< user:pass:1234:1234::/home/user:/bin/bash`
3. Check user line in /etc/shadow
```
/etc/shadow:user:HASH:19721:0:10000:7:::
```

Max days are set to 10000. Instead, this should be:

```
/etc/shadow:user:HASH:19721:0::7:::
```

Reviewed-by: Alejandro Colomar <alx@kernel.org>
Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>