Rework EC I/O approach. Implement burst mode, including proper handling of
case where it asynchronously exits burst mode on its own. Handle different
values of hz in sleep loop. Provide more debugging options to tune EC
behavior. These tunables/sysctls may be temporary and are not for user
access if the EC is working properly. Burst mode is now on by default for
testing and the poll interval has been increased from 100 to 500 us and
total timeout from 100 to 500 ms.

Hopefully this should be the first step of addressing reports of timeout
errors during battery or thermal access, especially on HP/Compaq laptops.
It is reasonably stable and should not cause a loss of functionality or
performance on systems that were previously working. Testing shows an
increase of responsiveness by ~75% on one system.

PR: kern/98171