1.0.24.39: mutex changes

1.0.24.39: mutex changes

- do what a FIXME suggests and rename MUTEX-VALUE to MUTEX-OWNER
- in the process, make sure that the value returned is less stale
- keep MUTEX-VALUE around for compatibility for a while
- also add HOLDING-MUTEX-P
- to make MUTEX-OWNER and HOLDING-MUTEX-P useful make unithread builds
  keep track of the owner of mutex