libbsc: Create pcu-socket only as specified in config file

Since commit b4999b60d48bcbb5aa575973d068e07ab672e095 we created PCU
sockets at hard-coded paths in the filesystem by default for all BTSs.
This is inflexible and prevents the use of multiple BSC instances on a
single filesystem, or the placement of the sockets in a more secure
location than /tmp.

The new approach with this patch is that
* no PCU sockets are created by default
* only for those BTSs where a 'pcu-socket' is configured via VTY,
  the socket will actually be created

Change-Id: Ie9079470584777dcc31f85f9bf0808f479156ccb
Closes: OS#2293
diff --git a/openbsc/include/openbsc/gsm_data_shared.h b/openbsc/include/openbsc/gsm_data_shared.h
index 3371fb9..1380a6c 100644
--- a/openbsc/include/openbsc/gsm_data_shared.h
+++ b/openbsc/include/openbsc/gsm_data_shared.h
@@ -870,6 +870,7 @@
 	struct amr_multirate_conf mr_half;
 
 	/* PCU socket state */
+	char *pcu_sock_path;
 	struct pcu_sock_state *pcu_state;
 
 #endif /* ROLE_BSC */