This fixed the issue that could cause the child process to stuck after fork.

This resolves #912.

This resolves #883.

Customized extent hooks may malloc / free thus trigger reentry.  Support this
behavior by adding reentrancy on hook functions.

Reported by Conrad Meyer.

Passing is_background_thread down the decay path, so that background thread
itself won't attempt inactivity_check.  This fixes an issue with background
thread doing trylock on a mutex it already owns.

This prevents signals from being inadvertently delivered to background
threads.

We use the minimal_initilized tsd (which requires no cleanup) for free()
specifically, if tsd hasn't been initialized yet.

Any other activity will transit the state from minimal to normal.  This is to
workaround the case where a thread has no malloc calls in its lifetime until
during thread termination, free() happens after tls destructors.

During purging, we may unlock decay->mtx.  Therefore we should finish logging
decay related counters before attempt to purge.

If neither background_thread nor lazy_lock is in use, do not abort on dlsym
errors.

This issue caused the default extent alloc function to be incorrectly
used even when arena.<i>.extent_hooks is set.  This bug was introduced
by 411697ad (Use exponential series to
size extents.), which was first released in 5.0.0.

We lookup freed extents for testing in post_reset.  Take background_thread lock
so that the extents are not modified at the same time.

Avoid calling pthread_create in postfork handlers.

To avoid complications, avoid invoking pthread_create "internally", instead rely
on thread0 to launch new threads, and also terminating threads when asked.

Also fix a compilation error #ifndef JEMALLOC_PTHREAD_CREATE_WRAPPER.

Pre-C11 compilers do not support typedef redefinition.

Avoid holding arenas_lock and background_thread_lock when creating background
threads, because pthread_create may take internal locks, and potentially cause
deadlock with jemalloc internal locks.

Since tsd cleanup isn't guaranteed when reincarnated, we set up tsd in a way
that needs no cleanup, by making it going through slow path instead.

It's possible to customize the extent_hooks while still using part of the
default implementation.

This makes sure we go down slow path w/ a0 in init.

The state initialization should be done before pthread_create.