…t threads

ocamldebug has never worked with multithreaded programs: multiple
threads mess up the communication protocol with the debugger, and the
fork()-based checkpointing mechanism breaks too.

Up to 4.11, the debugger would work fine until the first thread was
created.  Then, bizarre errors would be reported.

In 4.12 (commit e678885), a check was
added, causing any program linked with the threads library to report a
fatal error at start-up time when run under the debugger.

As reported in ocaml#10517, this check is too strong: some programs just
happen to be linked with the threads library, e.g. because they use
LWT, but may never start a thread.  There's no reason to refuse to
debug these programs.

This commit reverts the 4.12 change and adds a check in Thread.create
that aborts the program if it is run under the debugger.  This way,
programs linked with the threads library can still be debugged until
the point where they create new threads.