Write more redbean unit tests

- Fix DescribeSigset()
- Introduce new unix.rmrf() API
- Fix redbean sigaction() doc example code
- Fix unix.sigaction() w/ more than two args
- Improve redbean re module API (non-breaking)
- Enhance Lua with Python string multiplication
- Make third parameter of unix.socket() default to 0

tool/net/lfuncs.c

@@ -405,7 +405,7 @@ int LuaResolveIp(lua_State *L) {
 }
 
 static int LuaCheckControlFlags(lua_State *L, int idx) {
-  int f = luaL_checkinteger(L, idx);
+  int f = luaL_optinteger(L, idx, 0);
   if (f & ~(kControlWs | kControlC0 | kControlC1)) {
     luaL_argerror(L, idx, "invalid control flags");
     unreachable;