Clean up thread library a bit too.

data structures. Makes it easier to use pthreads too. Still need to add code for non-pthreads systems. Just a checkpoint to switch work to another machine.

various other little fixes.

Move libfmt, libutf into subdirectories of lib9. Add poll-based socket i/o to libthread, so that we can avoid using multiple procs when possible, thus removing dependence on crappy pthreads implementations. Convert samterm, acme to the single-proc libthread. Bring libcomplete, acme up-to-date w.r.t. Plan 9 distribution.

remove clumsy stack hack. fix exec. fix nthreads maintenance. fix threadexitsall not to kill self. add sun support.