add OSMO_IMSI_BUF_SIZE

Various places in our code base figure out how many chars they need to safely
store an IMSI. An IMSI can have a checksum digit, which is not reflected by
GSM23003_IMSI_MAX_DIGITS. And we usually need a terminating \0.

Instead of having a magic +2 repeated every so often, rather define
OSMO_IMSI_BUF_SIZE to contain both checksum digit and nul char, and have the
explanatory comment with it here in libosmocore.

Change-Id: Id11ada4c96b79f7f0ad58185ab7dbf24622fb770
diff --git a/include/osmocom/gsm/gsup.h b/include/osmocom/gsm/gsup.h
index 29ea11a..9a583aa 100644
--- a/include/osmocom/gsm/gsup.h
+++ b/include/osmocom/gsm/gsup.h
@@ -232,7 +232,7 @@
 /*! parsed/decoded GSUP protocol message */
 struct osmo_gsup_message {
 	enum osmo_gsup_message_type	message_type;
-	char				imsi[GSM23003_IMSI_MAX_DIGITS+2];
+	char				imsi[OSMO_IMSI_BUF_SIZE];
 	enum gsm48_gmm_cause		cause;
 	enum osmo_gsup_cancel_type	cancel_type;
 	int				pdp_info_compl;