[crypto] Add RFC 3526 FFDHE key exchange algorithms

[crypto] Add RFC 3526 FFDHE key exchange algorithms

The family of finite fields defined in RFC 7919 is almost identical to
that defined in RFC 3526, with the difference being that the older
standard uses the constant "pi" rather than "e".

Extend the definition of an FFDHE group to include a pointer to the
group constant, add the value of "pi", and define the modp2048,
modp3072, and modp4096 FFDHE groups.

Signed-off-by: Michael Brown <mcb30@ipxe.org>