Don't silently truncate numeric values

[legoscia]

If e.g. the message queue length for a process is greater than 999999, entop would silently truncate it and only display the first 6 digits. This patch makes it fill the field with asterisks instead, like io:format does when a value doesn't fit its field width.

[mazenharake]

Hi Magnus,

Thanks for your patch. I'll have a look as soon as I get time!