MINOR: tinfo: make thread_set functions return nth group/mask instead of first

thread_set_first_group() and thread_set_first_tmask() were modified
and renamed to instead return the number and mask of the nth group.
Passing zero continues to return the first one, but it will be more
convenient to use this way when building shards.
diff --git a/src/cfgparse.c b/src/cfgparse.c
index 06ee980..52f40d6 100644
--- a/src/cfgparse.c
+++ b/src/cfgparse.c
@@ -3016,8 +3016,8 @@
 					}
 
 					/* assign the first (and only) thread and group */
-					new_li->rx.bind_thread = thread_set_first_tmask(&new_ts);
-					new_li->rx.bind_tgroup = thread_set_first_group(&new_ts);
+					new_li->rx.bind_thread = thread_set_nth_tmask(&new_ts, 0);
+					new_li->rx.bind_tgroup = thread_set_nth_group(&new_ts, 0);
 					done -= todo;
 
 					shard++;
@@ -4458,8 +4458,8 @@
 
 					/* apply thread masks and groups to all receivers */
 					list_for_each_entry(li, &bind_conf->listeners, by_bind) {
-						li->rx.bind_thread = thread_set_first_tmask(&bind_conf->thread_set);
-						li->rx.bind_tgroup = thread_set_first_group(&bind_conf->thread_set);
+						li->rx.bind_thread = thread_set_nth_tmask(&bind_conf->thread_set, 0);
+						li->rx.bind_tgroup = thread_set_nth_group(&bind_conf->thread_set, 0);
 					}
 
 					if (bind_conf->xprt->prepare_bind_conf &&