As dev_t is now a 64-bit integer, it requires special handling as a
system call argument. 64-bit arguments are split between two 64-bit
integers due to the way arguments are promoted to allow reuse of most
system call implementations. They must be reassembled before use.
Further, 64-bit arguments at an odd offset (counting from zero) are
padded and slid to the next slot on powerpc and mips. Fix the
non-COMPAT11 system call by adding a freebsd32_mknodat() and
appropriately padded declerations.

The COMPAT11 system calls are fully compatible with the 64-bit
implementations so remove the freebsd32_ versions.

Use uint32_t consistantly as the type of the old dev_t matching the old
definition.