Use /usr/local/etc/ndn/nfd.conf by default
Currently, Mini-NDN uses
/usr/local/etc/ndn/nfd.conf.sample as a template when it creates NFD configuration files.
Instead, Mini-NDN should check if
/usr/local/etc/ndn/nfd.conf exists and if so, use it as the template.
/usr/local/etc/ndn/nfd.confis not installed automatically, Mini-NDN should use
/usr/local/etc/ndn/nfd.conf.sample as a backup in case it does not exist.
Mini-NDN copies the configuration file in bin/minindn