Fix dangerous casts in tests.

Fix dangerous casts of int variables to pointers in thread join function
calls.  On LP64 systems, int and pointers are different sizes, so
writes can corrupt memory.
Fix an off-by-one flaw in a test.
Fix a race condition in the "arenas.extend" mallctl.

Fix a race condition in the "arenas.extend" mallctl that could lead to
internal data structure corruption.  The race could be hit if one
thread called the "arenas.extend" mallctl while another thread
concurrently triggered initialization of one of the lazily created
arenas.
Fix a compiler warning.

Fix a compiler warning in chunk_record() that was due to reading node
rather than xnode.  In practice this did not cause any correctness
issue, but dataflow analysis in some compilers cannot tell that node and
xnode are always equal in cases that the read is reached.
Update README.
Update ChangeLog for 3.4.1.
Merge branch 'dev'
Prefer not_reached() over assert(false) where appropriate.
Consistently use malloc_mutex_prefork().

Consistently use malloc_mutex_prefork() instead of malloc_mutex_lock()
in all prefork functions.
Fix a data race for large allocation stats counters.

Reported by Pat Lynch.
Avoid (x < 0) comparison for unsigned x.

Avoid (min < 0) comparison for unsigned min in malloc_conf_init().  This
bug had no practical consequences.

Reported by Pat Lynch.
Add a missing mutex unlock in malloc_init_hard() error path.

Add a missing mutex unlock in a malloc_init_hard() error path (failed
mutex initialization).  In practice this bug was very unlikely to ever
trigger, but if it did, application deadlock would likely result.

Reported by Pat Lynch.
Fix a file descriptor leak in a prof_dump_maps() error path.

Reported by Pat Lynch.
malloc_ncpus(): fix Spc(s) mis-indentation to Tab(s)
malloc_ncpus() gets shorter