freebsd-update: mark "cron" as fetched as "fetch" itself

The change in 33bd05c3187d7b49c80cf1b0132b405c105d0833 was incomplete
because it did not mark "cron" as ISFETCHED=1 although it performs the
same operations as "install", but less output and does not perform a
hard exit. Mark result as such and make "install" know that updates have
been fetched.

PR: 277699
Approved by: jrm (mentor), emaste, cperciva
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D44356