Parameterize out the length of struct filed->f_lasttime as MAXDATELEN

This removes the hardcoded value for the field (16) and the equivalent
hardcoded lengths in logmsg(..).

This change is being done to help stage future work to add RFC5424/RFC5434
support to syslogd(8).

Obtained from: Isilon OneFS (dcd33d13da) (as part of a larger change)
Submitted by: John Bauman <john.bauman@isilon.com>
MFC after: 2 weeks
Sponsored by: Dell EMC Isilon