Some minor improvements to select/poll

[Andrew Doran <ad%netbsd.org@localhost>]

Hi all.

Long time no see.  For review:

(1) Increase the maximum number of clusters from 32 to 64 for large systems.
    kcpuset_t could potentially be used here but that's an excursion I don't
    want to go on right now.  uint32_t -> uint64_t is very simple.

(2) In the case of a non-blocking select/poll, or where we won't block
    because there are events ready to report, stop registering interest in
    the back-end objects early.

(3) Change the wmesg for poll back to "poll".

diff:
	http://www.netbsd.org/~ad/2019/select.diff

libmicro comparison:
	http://www.netbsd.org/~ad/2019/select.html

Any comments?

Andrew