Some optimizations for kqueue timers.

There is no need to do two allocations per kqueue timer. Gather all
data needed by the timer callout into the structure and allocate it at
once.

Use the structure to preserve the result of timer2sbintime(), to not
perform repeated 64bit calculations in callout.

Remove tautological casts.
Remove now unused p_nexttime [1].

Noted by: markj [1]
Reviewed by: markj (previous version)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
X-MFC note: do not remove p_nexttime
Differential revision: https://reviews.freebsd.org/D8901