Fix heap profiling bugs.

Fix a regression due to the recent heap profiling accuracy improvements:
prof_{m,re}alloc() must set the object's profiling context regardless of
whether it is sampled.

Fix management of the CHUNK_MAP_CLASS chunk map bits, such that all
large object (re-)allocation paths correctly initialize the bits.  Prior
to this fix, in-place realloc() cleared the bits, resulting in incorrect
reported object size from arena_salloc_demote().  After this fix the
non-demoted bit pattern is all zeros (instead of all ones), which makes
it easier to assure that the bits are properly set.