Set the tcb (thread control block) in the child process after a fork.

Set the tcb (thread control block) in the child process after a fork.
This protects against a race with an upcall in the parent during the
fork which can clobber the parent's tcb before the vm space is copied
in the child.  The child then gets a corrupted tcb that is either null
or that points to another thread that doesn't exist in the child (after
a fork, only the fork()ing thread exists in the child).

Reported by:	Arno J. Klaassen (arno at heho / snv / jussieu / fr)