New functions 'lua_resetthread' and 'coroutine.kill'

New functions to reset/kill a thread/coroutine, mainly (only?) to
close any pending to-be-closed variable. ('lua_resetthread' also
allows a thread to be reused...)

ldo.c

@@ -98,6 +98,10 @@ void luaD_seterrorobj (lua_State *L, int errcode, StkId oldtop) {
       setsvalue2s(L, oldtop, luaS_newliteral(L, "error in error handling"));
       break;
     }
+    case CLOSEPROTECT: {
+      setnilvalue(s2v(oldtop));  /* no error message */
+      break;
+    }
     default: {
       setobjs2s(L, oldtop, L->top - 1);  /* error message on current top */
       break;