Publish system status

[muep]

This is quite similar to #18, but for details that change over time. The best schedule for publishing this is not obvious. An initial idea might be to default to publishing these with an interval of one hour.

What this might cover:

• System RAM utilization
• System load average
• System real-time clock time
• CPU temperature
• CPU time and RAM utilization of sysmgr itself
• Mounted file systems and their space utilization

[muep]

CPU temperature was left for #23. File system status was left TBD