Upgrade Doug Lea's malloc to 2.8.6

Summary:
This brings in a few bug fixes and additions. The main changes are:
* Fail-fast: aborts instead of just printing a warning and ignoring
incorrect usage or memory corruption [!]
* Header file now exposes more routines and non-standard extensions, for
example aligned allocations and statistics.
* Adds `posix_memalign`, which I need for the Rust port.

Test Plan:
Ran small C and Rust programs manually but nothing thorough.

Differential Revision: https://code.systems.ethz.ch/D16

Signed-off-by: Zaheer Chothia <zchothia@inf.ethz.ch>

Document version, usage and modifications for external libraries

Note to others: please help adding and maintaining these notes. These
are tremendously helpful when comparing against and merging in changes
from new releases. As a user it also helps to be aware of Barrelfish-
specific quirks.