Our previous implementation of ProcessWatchdog was susceptible to a
well-known issue of GetProcessExitCode: if the process' real exit code
is STILL_ALIVE, then there's no way to determine that it has exited,
because this number is equal to a special value that function returns
for a living process.

This new implementation fixes that, and also introduces a new, better
API to pass more arguments (mostly for the purpose of testing).

LogLog.StoredRecords is a copy of the source list; clearing it has no
effect on the actual stored contents.

…timeout

timeout was behaving weirdly in cmd with IO redirection.

This code is out of range of valid exit codes on Unix anyway.