Honor NDN_CLIENT_TRANSPORT environment variable

[yoursunny]

Currently, NDNd applications are hard-coded to connect to /var/run/nfd/nfd.sock only.
They should be updated to honor NDN_CLIENT_TRANSPORT environment variable.

Benefits include:

• The applications can be configured to connect to an alternate forwarder instance.
• The applications can be configured to connect to NFD via localhost TCP, so that the traffic may be captured with tcpdump for debugging purposes.

[zjkmxy]

Doesn't the forwarder read unix.socket_path in the yaml file?

[pulsejet]

Forwarder we can leave as-is. For other apps a MakeDefaultFace function would do the trick.