Add a clock to the VGA warning

mostly so it doesn't immediately quit, but also so it's not eating CPU
cycles while being useless
Make serial devices more useful

They now use interrupts and thus can be blocked on without sitting in a
busy loop. Not sure if they still work perfectly, need to debug. Works
fine with the console "serial" stuff in qemu.

Todo: Investigate further.
Tasklets; fix dead sleep; kernel serial console

- Tasklets are essentially kernel threads. Still working on passing
  arguments to them, but they essentially just run functions and have
  special names like [[kttydebug]]. Eventually, I want disk scheduling
  and various (non-interrupt-driven) drivers running on these, but I'm
  still not sure how stable they are.
- Fix the scheduler so it supports not having anything to run. This took
  some tracking of what's running, and then inserting some liberal
  sleeps. Doesn't appear to break anything. Makes the system work when
  you try to sleep with only one process "running", so that's good.
- Start working on reimplementing the old kernel shell, but this time as
  a tasklet running in the background over serial. Probably going to try
  to add all the same features as before (tab completion, history, rich
  editing), but it may take some time to get it all in there. This
  console is mostly focused on helping with debugging EXT2 and other
  future stuff.
Add a 'headless' pseudo target
Bump version number