kernel - Be nicer to pthreads in vfork()
* When vfork()ing, give the new sub-process's lwp the same TID as the one
that called vfork(). Even though user processes are not supposed to do
anything sophisticated inside a vfork() prior to exec()ing, some things
such as fileno() having to lock in a threaded environment might not be
apparent to the programmer.
* By giving the sub-process the same TID, operations done inside the
vfork() prior to exec that interact with pthreads will not confuse
pthreads and cause corruption due to e.g. TID 0 clashing with TID 0
running in the parent that is running concurrently.