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
Reviewed by: allanjude, jilles
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D7747