If EVENT_MASK is given, then, if EVENT_MASK is non-zero, IO is added to the
list of file descriptors and the next call to poll will check for
any event specified in EVENT_MASK. If EVENT_MASK is zero then IO will be
removed from the list of file descriptors.
If EVENT_MASK is not given then the return value will be the current
event mask value for IO.
Returns a list of handles. If EVENT_MASK is not given then a list of all
handles known will be returned. If EVENT_MASK is given then a list
of handles will be returned which had one of the events specified by
EVENT_MASK happen during the last call ti poll