ctlstat: add prometheus output

When invoked by inetd, ctlstat -P will now produce output suitable for
ingestion into Prometheus.

It's a drop-in replacement for https://github.com/Gandi/ctld_exporter,
except that it doesn't report the number of initiators per target, and
it does report time and dma_time.

MFC after: 2 weeks
Sponsored by: Axcient
Relnotes: yes
Reviewed by: bapt, bcr
Differential Revision: https://reviews.freebsd.org/D29901