Make lush.interactive() return the result table in addition to setting _

Previously it only set the global _ and returned nothing, which was
surprising when calling it as a function. Now it returns the same
{code, stdout, stderr} table it sets _ to.

lcmd.c

@@ -1205,8 +1205,7 @@ int lushCmd_interactive (lua_State *L) {
   if (nstages > 1) {
     exec_pipeline(L, stages, nstages, 1);
     free(stages[0]);
-    lua_setglobal(L, "_");
-    return 0;
+    goto set_and_return;
   }
 
   /* single stage */
@@ -1220,21 +1219,22 @@ int lushCmd_interactive (lua_State *L) {
     dynbuf_init(&empty_out);
     dynbuf_init(&empty_err);
     push_result_table(L, 0, &empty_out, &empty_err);
-    lua_setglobal(L, "_");
-    return 0;
+    goto set_and_return;
   }
 
   /* check for shell builtin */
   if (try_builtin(L, &pa)) {
     free_argv(&pa);
-    lua_setglobal(L, "_");
-    return 0;
+    goto set_and_return;
   }
 
   fork_exec_single(L, &pa, 0);
   free_argv(&pa);
-  lua_setglobal(L, "_");
-  return 0;
+
+set_and_return:
+  lua_pushvalue(L, -1);    /* duplicate result table */
+  lua_setglobal(L, "_");   /* pop one copy into _ */
+  return 1;                 /* return the other */
 }