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/util/config-file-home/.ndn/client.conf b/tests/util/config-file-home/.ndn/client.conf
new file mode 100644
index 0000000..ba9f623
--- /dev/null
+++ b/tests/util/config-file-home/.ndn/client.conf
@@ -0,0 +1,2 @@
+a=/path/to/nowhere
+b=some-othervalue.01