Split kern_poll() on two counterparts.

The kern_poll_kfds() operates on clear kernel data, kfds points to an
array in the kernel, while kern_poll() operates on user supplied pollfd.
Move nfds check to kern_poll_maxfds().

No functional changes, it's for future use in the Linux emulation layer.

Reviewd by: kib
Differential Revision: https://reviews.freebsd.org/D30690
MFC after: 2 weeks