serpent, transform: rework GET_32BIT_MSB_FIRST, PUT_...

The macros GET_32BIT_MSB_FIRST and PUT_32BIT_MSB_FIRST duplicate
functionality available in unaligned.h, namely get_uint32 and
put_uint32.  Replace all uses outside serpent.c with calls to
get_uint32 and put_uint32.

We are going to make our serpent implementation compile-time
bytesexual, so we do retain a separate implementation of this
functionality in a form which will make that easier.  In particular,
they take arguments for the base and length of the array in
which 32-bit we are making the 32-bit access.

Also, this disentangles serpent.c from secnet.h.  To make serpent.c
easier to reuse, remove the #include of secnet.h (and replace it with
stdint.h, which we do need and were getting via secnet.h).

No functional change.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>