Until now the ifcfg-rh plugin merged the values of 'ipv4.dns-options'
and 'ipv6.dns-options' and wrote the result to the RES_OPTIONS
variable. This is wrong because writing a connection and reading it
back gives a different connection compared to the original.

This behavior existed since when DNS options were introduced, but it
became more evident now that we reread the connection after write,
because after doing a:

 $ nmcli connection modify ethie ipv4.dns-options ndots:2

the connection has both ipv4.dns-options and ipv6.dns-options set. In
order to delete the option, an user has to delete it from both
settings:

 $ nmcli connection modify ethie ipv4.dns-options "" ipv6.dns-options ""

To improve this let's use different variables for IPv4 and IPv6. To
keep backwards compatibility IPv4 still uses RES_OPTIONS, while IPv6
uses a new IPV6_RES_OPTIONS variable.

https://bugzilla.redhat.com/show_bug.cgi?id=1517794