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/client.conf.sample b/client.conf.sample
new file mode 100644
index 0000000..6cde2eb
--- /dev/null
+++ b/client.conf.sample
@@ -0,0 +1,10 @@
+; "unix_socket" specifies the location of the NFD unix socket
+unix_socket=/var/run/nfd.sock
+
+; "protocol" deteremines the protocol for prefix registration
+; it has a value of:
+;   nfd-0.1
+;   nrd-0.1
+;   ndnd-tlv-0.7
+;   ndnx-0.7
+protocol=nrd-0.1
\ No newline at end of file