Move socket file to /run on Linux
/var/run  is considered legacy on Linux, the replacement is
/run . systemd v239+ started complaining about services and other unit files using
/var/run. We should migrate to the new path, and ndn-cxx should look for the socket file in
/run first (on Linux only).
On most, if not all, modern Linux distros
/var/run is a symbolic link to
/run, so I don't expect any disruption.