Implemented UserFile file descriptor casting.

Implemented UserFile file descriptor casting by calling user defined function streamWrapper::stream_cast(int). Also in the function which converts File* to file descriptor, checks whether the casting function fd() returns -1, raise warning if so. Finally after all the castings for read / write / except array are done, check whether any file descriptors are successfully casted, if not, raise warning and return false.

Passing vfsstream selectStream test.