Add OpenVPN probe

This adds support for OpenVPN status probe which uses OpenVPN's
management interface (configured via 'management 127.0.0.1 1234' in
OpenVPN's config).

The output looks as follows:
...
  OpenVPN
    127.0.0.1:1234
      status: CONNECTED
      tunnel: 10.8.0.15
      remote: 144.76.43.77:1194
    localhost:4242
      status: management interface incompatible
    127.0.0.1:4444
      status: management interface unavailable
...

We show tunnel's IP (if available) as well as remote (OpenVPN server
itself) address/port in addition to general connection status. If
management interface is unavailable it's reported as such. If we've
managed to establish connection with a given management interface but
are unable to obtain expected information than we report this
incompatibility as well.

Related: SYS#2655
Change-Id: I4493e19b9a09dcebd289457eacd1719f7f8cc31c
diff --git a/src/Makefile.am b/src/Makefile.am
index f639023..f9b79f2 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -9,6 +9,7 @@
 	-Wall \
 	$(LIBOSMOCORE_CFLAGS) \
 	$(LIBOSMOGSM_CFLAGS) \
+	$(LIBOSMONETIF_CFLAGS) \
 	$(NULL)
 
 AM_LDFLAGS = \
@@ -22,12 +23,13 @@
 
 noinst_LTLIBRARIES = libintern.la
 libintern_la_SOURCES = simple_ctrl.c client.c
-libintern_la_LIBADD = $(LIBOSMOCORE_LIBS) $(LIBOSMOGSM_LIBS)
+libintern_la_LIBADD = $(LIBOSMOCORE_LIBS) $(LIBOSMOGSM_LIBS) $(LIBOSMONETIF_LIBS)
 
 osmo_sysmon_CFLAGS = $(LIBMNL_CFLAGS) $(LIBOSMOVTY_CFLAGS) $(LIBOPING_CFLAGS) $(AM_CFLAGS)
 
 osmo_sysmon_LDADD = $(LDADD) \
 	$(LIBOSMOVTY_LIBS) \
+	$(LIBOSMONETIF_LIBS) \
 	$(LIBMNL_LIBS) \
 	$(LIBOPING_LIBS) \
 	$(NULL)
@@ -39,6 +41,7 @@
 	osysmon_rtnl.c \
 	osysmon_file.c \
 	osysmon_ping.c \
+	osysmon_openvpn.c \
 	osysmon_main.c \
 	$(NULL)