Add posix_memalign(), fix minor bug in nmalloc.

Add posix_memalign(), fix minor bug in nmalloc.

Add the posix_memalign() function in all of its glory.  Our new slab
allocator already does most of the job perfectly, particularly when
alignment < size (for things like cache-line aligned allocations).

Correct a bug in _vmem_alloc() for the case where (size) is much larger
then (alignment).  The hack to get mmap() to return an aligned address
was not properly unmapping temporarily-mapped space.

Reformulate how errno is set to support posix_memalign(), which is defined
by the standard to return the error rather then set errno.

Requested-by: Hasso Tepper <hasso@estpak.ee>