Named return variables should only be used to describe the returned type
further, e.g. `err error` doesn't add any new information and is just
stutter.
Remove all hardcoded references to `/proc`. For all collectors that do
not use `github.com/prometheus/procfs` yet, provide a wrapper to
generate the full paths.
Reformulate help strings, errors and comments to remove absolute
references to `/proc`.
This is a breaking change: the `-collector.ipvs.procfs` flag is removed
in favor of the general flag. Since it only affected that collector it
was only useful for development, so this should not cause many issues.
