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_hlr.c b/src/db_hlr.c
index 9c74a24..fa962f3 100644
--- a/src/db_hlr.c
+++ b/src/db_hlr.c
@@ -37,8 +37,8 @@
 			x[sizeof(x)-1] = '\0';				\
 		} while (0)
 
-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)
 {
 	sqlite3_stmt *stmt = dbc->stmt[DB_STMT_SEL_BY_IMSI];
 	int rc;