Acquire/release arena bin locks as part of the prefork/postfork. This
bug made deadlock in the child between fork and exec a possibility.
Split jemalloc_postfork() into jemalloc_postfork_{parent,child}() so
that the child can reinitialize mutexes rather than unlocking them. In
practice, this bug tended not to cause problems.
When jemalloc is used as a libc malloc replacement (i.e. not prefixed),
some particular setups may end up inconsistently calling malloc from
libc and free from jemalloc, or the other way around.
glibc provides hooks to make its functions use alternative
implementations. Use them.
Submitted by Karl Tomlinson and Mike Hommey.
