MFS r305423: cron: use existing maximum username constant MAXLOGNAME

Previously cron had its own maximum username length limit, which was
smaller than the system's MAXLOGNAME. This could lead to crontab -u
updating the wrong user's crontab (if the name was truncated, and
matched another user).

PR: 212305
Reported by: Andrii Kuzik
Approved by: re (kib)
MFH: r305269