847ff22 added a call to malloc_default_zone() before the main loop in
register_zone, effectively making malloc_default_zone() called twice
without any different outcome expected in the returned result.

It is also called once at the beginning, and a second time at the end
of the loop block.

Instead, call it only once per iteration.

On OSX 10.12, malloc_default_zone returns a special zone that is not
present in the list of registered zones. That zone uses a "lite zone"
if one is present (apparently enabled when malloc stack logging is
enabled), or the first registered zone otherwise. In practice this
means unless malloc stack logging is enabled, the first registered
zone is the default.

So get the list of zones to get the first one, instead of relying on
malloc_default_zone.

Fix for rust-lang/rust#34674

Fix zone_force_unlock() to reinitialize, rather than unlocking mutexes,
since OS X 10.12 cannot tolerate a child unlocking mutexes that were
locked by its parent.

Refactor; this was a side effect of experimenting with zone
{de,re}registration during fork(2).

The SDK jemalloc is built against might be not be the latest for various
reasons, but the resulting binary ought to work on newer versions of
OSX.

In order to ensure this, we need the fullest definitions possible, so
copy what we need from the latest version of malloc/malloc.h available
on opensource.apple.com.

…ions

Some system libraries are using malloc_default_zone() and then using
some of the malloc_zone_* API. Under normal conditions, those functions
check the malloc_zone_t/malloc_introspection_t struct for the values
that are allowed to be NULL, so that a NULL deref doesn't happen.

As of OSX 10.12, malloc_default_zone() doesn't return the actual default
zone anymore, but returns a fake, wrapper zone. The wrapper zone defines
all the possible functions in the malloc_zone_t/malloc_introspection_t
struct (almost), and calls the function from the registered default zone
(jemalloc in our case) on its own. Without checking whether the pointers
are NULL.

This means that a system library that calls e.g.
malloc_zone_batch_malloc(malloc_default_zone(), ...) ends up trying to
call jemalloc_zone.batch_malloc, which is NULL, and crash follows.

So as of OSX 10.12, the default zone is required to have all the
functions available (really, the same as the wrapper zone), even if they
do nothing.

This is arguably a bug in libsystem_malloc in OSX 10.12, but jemalloc
still needs to work in that case.

Update zone allocator with various OSX 10.12 fixes