UKERNEL: allow creation of non-detached threads

UKERNEL: allow creation of non-detached threads

Make usr_thread_create create a non-detached thread by default, and
just have callers call usr_thread_detach if they want it detached. The
only current caller of usr_thread_create already calls
usr_thread_detach.

Also add usr_thread_join to make it possible to join a created thread.

Change-Id: Iad581c45d36dc43e94c950a5ca6a41dea8dc4b2f
Reviewed-on: http://gerrit.openafs.org/1722
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>