Add libctrl, an SNMP-like control interface

In contrast to the VTY interface the control interface is meant to be
used by programs.
This patch adds basic support, no commands are defined.
diff --git a/openbsc/configure.ac b/openbsc/configure.ac
index 5ed5303..89cfcac 100644
--- a/openbsc/configure.ac
+++ b/openbsc/configure.ac
@@ -92,6 +92,7 @@
     src/libtrau/Makefile
     src/libabis/Makefile
     src/libbsc/Makefile
+    src/libctrl/Makefile
     src/libmsc/Makefile
     src/libmgcp/Makefile
     src/libcommon/Makefile