Make Thread a wrapper for both ithreads and 5005threads.

Make Thread a wrapper for both ithreads and 5005threads.
Needs a lot of work and Arthur's eyeballing (and finishing
few missing odds and ends of threads.pm).  Now Thread
is an extension without its own .pm.

If there are people who have experience working with
either the 5005threads or the ithreads: cleaning up
the documentation (Thread.pm, threads.pm, and perl*thrtut.pod),
and porting the various Thread::* modules, like Thread::Queue,
to work also with ithreads, is needed.

Removed the 5005threads requirement from the 5005 tests
for easier testing (not all of them work yet since
they use the Thread::* modules that have not been made
to work with ithreads).  Leave the 5005 tests named as
*.tx so that they are not run by "make test".

p4raw-id: //depot/perl@13036