sysctl: consider --prefix while parsing the files

sysctl: consider --prefix while parsing the files

not while applying the parsed sysctl values. Otherwise
info "Overwriting earlier assignment of %s in file %s" is
visible many times even though the given --prefix doesn't
try to set the overridden value.

This also optimizes the startup tiny bit since we have udev
rules running on network devices and setting sysctl through
the rules.