Fix in place huge reallocation to update the chunk counters that are
used for triggering gdump profiles.

Fix xallocx(..., MALLOCX_ZERO to zero the last full trailing page of
large allocations that have been randomly assigned an offset of 0 when
--enable-cache-oblivious configure option is enabled.  This addresses a
special case missed in d260f442 (Fix
xallocx(..., MALLOCX_ZERO) bugs.).

Work around a potentially bad thread-specific data initialization
interaction with NPTL (glibc's pthreads implementation).

This resolves #283.

Zero all trailing bytes of large allocations when
--enable-cache-oblivious configure option is enabled.  This regression
was introduced by 8a03cf03 (Implement
cache index randomization for large allocations.).

Zero trailing bytes of huge allocations when resizing from/to a size
class that is not a multiple of the chunk size.

Fix prof_tctx_dump_iter() to filter out nodes that were created after
heap profile dumping started.  Prior to this fix, spurious entries with
arbitrary object/byte counts could appear in heap profiles, which
resulted in jeprof inaccuracies or failures.

Fix prof_alloc_rollback() to read tdata from thread-specific data rather
than dereferencing a potentially invalid tctx.

Fix irallocx_prof_sample() to always allocate large regions, even when
alignment is non-zero.

Fix ixallocx_prof_sample() to never modify nor create sampled small
allocations.  xallocx() is in general incapable of moving small
allocations, so this fix removes buggy code without loss of generality.