• Extend the condvar(9) KPI introducing a new cv_timedwait_bt_sig()

function so that we can specify timeout precision in terms of struct
bintime.

• Now seltdwait() takes three argument rather than two so that their

consumers can specify if the timeout should be passed as ticks or bintime.

• Refactor the kern_select() and the sys_poll() code so that these two

services may rely on cv_timedwait_bt_sig() rather than on the previous less
precise cv_timedwait_sig().

• Rethink the sleepqueue(9) KPI in order to make an attempt of avoiding

both code duplication and breakages. Your mileage WILL vary, feel free to
comment.

Reviewed by: mav