Hack on the newly imported X25519 and X448 code.

Hack on the newly imported X25519 and X448 code.

I've tried to touch the code as little as possible to integrate it with
the rest of Secnet.  I've limited myself to:

  * adding Secnet notices and GPL-mandated notices;

  * removing definitions which obviously won't work;

  * removing code which is unnecessary given Secnet's less ambitious
    portability objectives; and

  * providing substitutes for some of the support types and macros
    assumed by the original implementation, in particular a fake version
    of <mLib/bits.h>.

I've also written a new test driver for the field and curve arithmetic,
and reformatted the test vectors, though the data remains the same.

The code builds and passes the tests.

Signed-off-by: Mark Wooding <mdw@distorted.org.uk>