util, transport: add configuration file support and make default unix socket configurable

add sample library configuration file

refs: #1364

Change-Id: I3cb36d078aa3f0b0a50d9a83a521e95448df0a93
diff --git a/tests/transport/test-homes/ok/.ndn/client.conf b/tests/transport/test-homes/ok/.ndn/client.conf
new file mode 100644
index 0000000..e2d3556
--- /dev/null
+++ b/tests/transport/test-homes/ok/.ndn/client.conf
@@ -0,0 +1,2 @@
+
+unix_socket=/tmp/test/nfd.sock