sd-netlink: prefix output param names with 'ret'

sd-netlink: prefix output param names with 'ret'

In some places, the code was already using that, so only the header file needed
to be adjusted.

We use 'nl' for the sd_netlink* when it's passed in as input. I was considering
renaming to 'netlink', but that'd add a lot of verbosity and also we have
'sd_journal *j' in sd-journal, so the short name matches in style.