Our kern_sigtimedwait() calculates absolute sleep timo value as 'uptime+timeout'.
So, when the user specifies a big timeout value (LONG_MAX), the calculated
timo can be less the the current uptime value.
In that case kern_sigtimedwait() returns EAGAIN instead of EINTR, if
unblocked signal was caught.
While here switch to a high-precision sleep method.
Below are to separate commits: fix and tests for it and for some other
kib@ commits to sigwait are.