* the first one would cause an unnecessary malloc() of iovec storage for
a msg_iovlen of UIO_SMALLIOV although the required amount of memory has
been allocated on the stack.
* the second one would cause a recvmsg() or sendmsg() with a msg_iovlen of
UIO_MAXIOV to fail with EMSGSIZE, which is also a violation of XNS5.
- Created compat_util.c and compat_util.h to be used by the compatibility
modules, so they don't duplicate the same code.
- Added prototypes to the stackgap allocation routines.
