Address several issues regarding the psi-plugin (pressure stall information).
Fixes:
- Use local for variables in functions.
- Add fetch as a valid parameter and remove auto.
- Remove double slash in path for get_pressure_value() and quote it.
- Remove line break in return value of get_printable_name().
- Quote variables to avoid splitting/globbing.
- Rename pressure plugin to linux_psi.
References:
- https://github.com/munin-monitoring/contrib/pull/1302
