All interior pointers must be enabled for finalized allocation.

* include/gc_disclaim.h: Add note about all-interior-pointers requirement.
* fnlz_mlc.c (GC_init_finalized_malloc): Abort with a useful message if all
interior pointers are not enabled.
* tests/disclaim_bench.c, tests/disclaim_test.c: Enable all interior
pointers for tests.
Revert "All interior pointers must be enabled for finalized allocation."
(Ensuring all-interior-pointers mode is no longer needed.)

This reverts commit 97f113395e228567eec1f9105bccdd7d3db6c66e.
Merge (dummy) branch 'fnlz-requires-all-interior'
Add weak stubs for pthread_cancel API.
Adjust 'pthread_[un]register_cancel undefined ref' workaround (Pthreads)

* misc.c (__pthread_register_cancel, __pthread_unregister_cancel):
Move prototypes to pthread_support.c.
* pthread_support.c (__pthread_unregister_cancel,
__pthread_unregister_cancel): Declare only if
PTHREAD_REGISTER_CANCEL_WEAK_STUBS defined.