FD is a poller file-descriptor. Typically a file-descriptor connected to a terminal, a pseudo terminal, a character device, a FIFO (named pipe), or an

fdCtl keeps control fields (locks, timers, etc) for a single direction. For every FD there is one fdCtl for Read operations and another for Write oper

TODO(npat): Perhaps lock fdMap with a RWMutex?? fdMap maps IDs (arbitrary integers) to FD structs. IDs are *not* reused, so if we get stale file-descr

selectCtx contains variables used to interact with the select goroutine.