S390 options are stored in a separate [ethernet-s390-options] section.
This group must not be interpreted as a NMSetting name, otherwise we
log a bogus warning:
<warn> [1590523563.7757] keyfile: ethernet-s390-options: invalid setting name 'ethernet-s390-options'
Fixes: cf9b8d3bad ('libnm/keyfile: implement ethernet.s390-options in keyfile')