poll: Unify userspace pollfd pointer name

Some of the poll code used 'fds' and some used 'ufds' to refer to the
uap->fds userspace pointer that was passed around to subroutines. Some of
the poll code used 'fds' to refer to the kernel memory pollfd arrays, which
seemed unnecessarily confusing.

Unify on 'ufds' to refer to the userspace pollfd array.

Additionally, 'bits' is not an accurate description of the kernel pollfd
array in kern_poll, so rename that to 'kfds'. Finally, clean up some logic
with mallocarray() and nitems().

No functional change.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D17670