New log_max_line_length option.

Log lines can get quite large, as we previously noticed with rsync error
log lines. We added a setting to cap those, but it really looks like we
should have just done this overall limit. We noticed the issue when we
switched to UDP syslogging and it would occasionally blow past the 16436
lo MTU! This causes Python's logging code to get an error and hilarity
ensues.

Change-Id: I44bdbe68babd58da58c14360379e8fef8a6b75f7