Rename bsc_nat -> osmo-bsc_nat and bsc_mgcp -> osmo-bsc_mgcp

This now enforces a unique structure: All of our main daemon
programs start with an "osmo-" prefix.
diff --git a/openbsc/configure.in b/openbsc/configure.in
index 525bad8..65b39e6 100644
--- a/openbsc/configure.in
+++ b/openbsc/configure.in
@@ -95,7 +95,8 @@
     src/common/Makefile
     src/osmo-nitb/Makefile
     src/osmo-bsc/Makefile
-    src/nat/Makefile
+    src/osmo-bsc_nat/Makefile
+    src/osmo-bsc_mgcp/Makefile
     src/ipaccess/Makefile
     src/utils/Makefile
     src/gprs/Makefile