poll/select: Use 64bit serial for poll/select's kevent.udata.

poll/select: Use 64bit serial for poll/select's kevent.udata.

This fixes the issue mentioned in this commit:
ce4975442fa0524017fb3c1aef93bbe6880ae770

It takes ~200 years for 2.5Ghz cpu to make the 64bit serial wrap,
even if the cpu's speed were 10 times faster tomorrow, it still
would take two decades to make the 64bit serial wrap.

Suggested-by: dillon@