Method i_err_disable_cause implemented on CiscoPortSecurity.pm. Returns
sparse data: ifIndex -> textual err-disable cause, for interfaces that
are err-disabled.
Serial numbers weren't reported for stackable Layer3::H3C devices, even
when not actually stacked. Report serial number not only if the entity
has no parent, but also if it has a parent but is of type "chassis".
This fixes#232.
Data rates up to MCS 15 (two spatial streams) at 20 MHz and 40 MHz were
defined; added MCS 16 through 23.
Decoding MCS rates at SS1 and SS2 not implemented yet.
