AMD64 - Refactor uio_resid and size_t assumptions.

AMD64 - Refactor uio_resid and size_t assumptions.

* uio_resid changed from int to size_t (size_t == unsigned long equivalent).

* size_t assumptions in most kernel code has been refactored to operate in a
  64 bit environment.

* In addition, the 2G limitation for VM related system calls such as mmap()
  has been removed in 32 bit environments.  Note however that because
  read() and write() return ssize_t, these functions are still limited
  to a 2G byte count in 32 bit environments.