Introduce libsmpputil

As part of preparation for libosmo-netif migration let's move common SMPP code
into separate build-time library and use it for both smpp_mirror and OsmoMSC
renaming the files if necessary.

While at it we also fix id/password legth limits in smpp_mirror and drop unused
fields from ESME struct.

Related: OS#5568
Change-Id: I61910651bc7c188dc2fb67d96189a66a47e7e8fb
diff --git a/src/libmsc/gsm_04_11.c b/src/libmsc/gsm_04_11.c
index 743e272..adc9d88 100644
--- a/src/libmsc/gsm_04_11.c
+++ b/src/libmsc/gsm_04_11.c
@@ -58,7 +58,7 @@
 #include <osmocom/msc/paging.h>
 
 #ifdef BUILD_SMPP
-#include "smpp_smsc.h"
+#include <osmocom/smpp/smpp_smsc.h>
 #endif
 
 void *tall_gsms_ctx;
@@ -1389,4 +1389,3 @@
 		trans_free(trans);
 	}
 }
-