Fixed various problems with command-dot handling (some very old):

Fixed various problems with command-dot handling (some very old):
- Don't scan for cmd-. unless in the foreground
- Scan before switching out to other processes, not after
- don't scan if SchedParams.check_interrupt is false (!)
  - But: do scan if we're blocked on I/O

One problem remains: in the last case KeyboardInterrupt is raised
too late.