SHMFS support for reading dstate (simplify monitoring)

[jimklimov]

Current monitoring integrations to represent NUT data points for SNMP scripted agents, MRTG collectors, etc. rely on running upsc in scripts, often once per data point. All this process forking comes at a cost.

We should consider exposing the readings (R/O at least, since they are public info for protocol-based tools without extra auth) in a shared-memory filesystem endpoint, similar to Linux use of /proc and /sys.

Challenges:

• See if similar facilities exist for other OSes (BSD, illumos, ...)
• Should this be an option for upsd itself, reflecting changes as soon as they come, or a separate persistent client similar to upsmon which could so expose remote devices, take care of R/W value setting while honouring NUT user accounts and SSL, etc.? Maybe both, in different namespaces...

[jimklimov]

In case of separate client, another close match is upslog

See also http://bugs.debian.org/343530 about a version enhanced for rrdtool compatible logging.