Do some work on redbean

- Rewrite Slurp() API to be like string.sub()
- Introduce a new Barf() API for creating files
- Update Redbean `-S` sandbox flag to do unveiling

tool/net/lfuncs.h

@@ -10,6 +10,7 @@ int LuaRe(lua_State *);
 int luaopen_argon2(lua_State *);
 int luaopen_lsqlite3(lua_State *);
 
+int LuaBarf(lua_State *);
 int LuaBenchmark(lua_State *);
 int LuaBin(lua_State *);
 int LuaBsf(lua_State *);