BUGS/Restrictions:
-----------------
- assertions were not safe in 4.2, fixed in 4.3.
- only flat objects can be shared safely between threads (all versions)
- if an object with reference in it is put into a proxy, the sub-references
can be removed by the GC, even if it is still referenced. (see workaround in
producer_consumer example)
- EiffelNet was not safe with 4.2, fixed in 4.3 (see mt_two_mach in mtnet 
directory)
- vision is not safe with eiffelthread on Unix (at least with non MT-safe
Motif library 1.x and 2.0)
- Once functions are per thread. Once per process functions implemented
in ONCE_CONTROL only concern flat object.

Changes 
-------

4.2 release:
new classes CONDITION VARIABLE, THREAD_ATTRIBUTES, SEMAPHORE,
OBJECT_OWNER, ONCE_CONTROL.

PROXY is no longer expanded.

4.3 release:
get_current_id, exit, last_created_thread imoved from THREAD to
THREAD_CONTROL

put/make in PROXY is {NONE}