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/src/Makefile.am b/openbsc/src/Makefile.am
index df7b936..c7de811 100644
--- a/openbsc/src/Makefile.am
+++ b/openbsc/src/Makefile.am
@@ -2,7 +2,7 @@
 AM_CFLAGS=-Wall $(LIBOSMOCORE_CFLAGS) $(LIBOSMOGSM_CFLAGS) $(LIBOSMOGSM_CFLAGS) $(LIBOSMOVTY_CFLAGS) $(COVERAGE_CFLAGS)
 AM_LDFLAGS = $(LIBOSMOCORE_LIBS) $(LIBOSMOGSM_LIBS) $(COVERAGE_LDFLAGS)
 
-SUBDIRS = libcommon libabis libmgcp libbsc libmsc libtrau osmo-nitb osmo-bsc_mgcp utils ipaccess libgb gprs
+SUBDIRS = libcommon libabis libmgcp libbsc libmsc libtrau libctrl osmo-nitb osmo-bsc_mgcp utils ipaccess libgb gprs
 
 # Conditional modules
 if BUILD_NAT