Next round of serial low-latency work.
- Remove serial_run() from everywhere, add it to a SIGALRM handler. With the
async notifications, that should be enough.
- Remove serial throttling hack - it doesn't play well with the async IO.
- Make sure the ASYNC_LOW_LATENCY flag is set.


git-svn-id: https://dosemu.svn.sourceforge.net/svnroot/dosemu/trunk@294 7758b857-130e-0410-ab9c-d8735a528d18