valloc - aligned memory allocator
Valloc allocates size bytes aligned on a page boundary. It is
implemented by calling malloc(3) with a slightly larger request,
saving the true beginning of the block allocated, and returning a
properly aligned pointer.
Valloc returns a null pointer (0) if there is no available memory
or if the arena has been detectably corrupted by storing outside
the bounds of a block.
Vfree isn’t implemented.