Filesystem/pgsql: silence spurious WARNING and ERROR log messages

[lge]

• pgsql: pgsql_status: silence ERROR: command failed: runuser
• Filesystem: for "probe", silence "Expected /dev/... to exist"

[lge]

Updated to include the first commit from #1950 and folded the second commit (pgsql runasowner debug support) into this one.

[oalbrigt]

Thanks.