We rely on the include path to find many of these headers. It
especially doesn't make sense to use #include "foo.h" for headers
in the top-level include directory.
These allocate memory via mmap anonymous regions and store the mapped
size immediately before the returned pointer as an unsigned long.
They are intended to be used in cases where malloc(3) and free(3)
are unsuitable due to concerns about corrupting global state in
multi-threaded programs or signal handlers.
