Let the config parser make use of the mib_string2oid().

Let the config parser make use of the mib_string2oid().
If a descriptor is not found in the loaded MIB files it falls back to
the old smi_oid2string(), which then throws a deprecation warning. This
won't trigger for most cases in the default install, but the
UCD-DISKIO-MIB and dependencies aren't included (yet?) (which can be
fixed by manually including them via "mib directory") and there's a
couple of misspellings (e.g. mib_2 vs mib-2, and
usmStatsNotInTimeWindow vs usmStatsNotInTimeWindows).

Feedback and OK tb@