set CHANGE replace LY_SET_OPT_USEASLIST by a single flag in ly_set_add()

There was just a single option and the ly_set API is quite stable for a
long time, so there will be hardly any new option. So instead of an
option bitmask with a single value, make the ly_set_add() parameter only
a single flag to switch off checking (and ignoring) items already
present in the set.
diff --git a/src/context.c b/src/context.c
index 85ae0d1..6b06acc 100644
--- a/src/context.c
+++ b/src/context.c
@@ -97,7 +97,7 @@
                 return LY_EEXIST;
             }
         }
-        if (ly_set_add(&ctx->search_paths, new_dir, LY_SET_OPT_USEASLIST, NULL)) {
+        if (ly_set_add(&ctx->search_paths, new_dir, 1, NULL)) {
             free(new_dir);
             return LY_EMEM;
         }