This trivial patch causes the TLS to be cleared on execve (code is in
flush_thread).  This is necessary to avoid ESRCH errors when
set_thread_area is asked to choose a free TLS entry after several nested
execve's.

The LDT also has a similar problem, but it is less serious because the
LDT code doesn't scan for free entries.  I'll probably send a patch to
fix this too, unless there is something important relying on this
behavior.