Introduce '-D' commandline option to daemonize processes

This uses the osmo_daemonize() function of libosmocore >= 0.1.18,
and is now implemented for bac_nat, osmo-bsc, bsc_hack, osmo-gbproxy
and bsc_mgcp.  This means only osmo-sgsn is missing, which currently
has no option parsing at all.
diff --git a/openbsc/configure.in b/openbsc/configure.in
index c683f76..0167725 100644
--- a/openbsc/configure.in
+++ b/openbsc/configure.in
@@ -43,7 +43,7 @@
     ])
 AM_CONDITIONAL(BUILD_BSC, test "x$osmo_ac_build_bsc" = "xyes")
 
-PKG_CHECK_MODULES(LIBOSMOCORE, libosmocore >= 0.1.16)
+PKG_CHECK_MODULES(LIBOSMOCORE, libosmocore >= 0.1.18)
 PKG_CHECK_MODULES(LIBOSMOVTY, libosmovty >= 0.1.9)
 
 dnl checks for header files