Allow fractional values of `polling-period'

Allow fractional values of `polling-period'

This allows C-g to be handled faster in the NS port at the cost
of some extra CPU time on slow machines.

* etc/NEWS: Announce new feature.
* src/keyboard.c (start_polling):
(bind_polling_period): Handle floating point values of
`polling-period'.
(syms_of_keyboard): Make `polling-period' a Lisp variable
instead of an int variable.