Update to 3.1.7-15-ge84bac5.

I had intended to update this to master but have not yet fixed a blocking
bug on it.

Changes:

• status -a: Fix handling of showing older builds.
• QEMU: Add a hack to raise the default build times.
• Don't crash starting up a job, or otherwise, if the time goes backwards.
• blacklist: Fix loading of JAIL-SETNAME-blacklist
• QEMU: Always try loading a qemu-blacklist file
• Add a helper 'clock_monotonic' to use clock_gettime(CLOCK_MONOTONIC) rather than non-monotonic 'date +%s'
• Use monotonic time for calculating durations
• Add a build.started epoch time.
• Add a build.ended epoch time that is set in the last HTML json generation.
• Update sh to FreeBSD r281982
• Fix build on older FreeBSD due to sh. If F_DUPFD_CLOEXEC is unavailable then use system sh.
• Ignore /var/db/freebsd-update/ too.