mon: die if 'mon data' fs has critically low available disk space & fix logging issues

Bunch of commits on the same PR, not necessarily related.

First batch prevents the monitor from starting if the disk is critically full.

Second batch will fix some logging channel issues:

- fix unexpanded $channel meta variables in log file names
- make sure to always default to 'cluster' if channel is not defined on
  LogEntry (which will happen if we are running a mixed-version cluster
  with entities that do not understand channels)
- log read-only commands with 'debug' level instead of 'info'.  Only
  read-write and commands requiring 'execute' permissions (regardless
  of 'read-only' or 'read-write') will be logged on 'info'.

Fixes: #9455, #9502

Signed-off-by: Joao Eduardo Luis ``<joao@redhat.com>``