cosmetic: rename db_subscr_get() to db_subscr_get_by_imsi()

There will be more additions, _by_msisdn() and _by_id(), to serve the upcoming
VTY commands, to allow flexibly selecting subscribers as in the old OsmoNITB.

Change-Id: I32fa676ccc5c10eba834c4390c8a42476b9c1961
diff --git a/src/db.h b/src/db.h
index d424b38..0064a4d 100644
--- a/src/db.h
+++ b/src/db.h
@@ -71,8 +71,8 @@
 	bool		ms_purged_ps;
 };
 
-int db_subscr_get(struct db_context *dbc, const char *imsi,
-		  struct hlr_subscriber *subscr);
+int db_subscr_get_by_imsi(struct db_context *dbc, const char *imsi,
+			  struct hlr_subscriber *subscr);
 int db_subscr_ps(struct db_context *dbc, const char *imsi, bool enable);
 int db_subscr_lu(struct db_context *dbc,
 		 const struct hlr_subscriber *subscr,