Enable both sbrk(2)- and mmap(2)-based memory acquisition methods by

Enable both sbrk(2)- and mmap(2)-based memory acquisition methods by
default.  This has the disadvantage of rendering the datasize resource
limit irrelevant, but without this change, legitimate uses of more
memory than will fit in the data segment are thwarted by default.

Fix chunk_alloc_mmap() to work correctly if initial mapping is not
chunk-aligned and mapping extension fails.