There is no upside in sharing pollers between application processes,
since file descriptors are process-local handles in essence. Therefore
pollers are process-local utilities, so we don't need them to be
visible from the file system.

Downgrade the poller element to a plain single device from the
factory. Each open instance of such device is a polling channel. At
this chance, do a general rename from poller to poll, to match
commonly accepted naming.

Signed-off-by: Philippe Gerum <rpm@xenomai.org>