[crypto] Add RFC 7919 FFDHE key exchange algorithms

We currently support fully parameterized finite field Diffie-Hellman
key exchange, where the peer provides not only its public key but also
the (fully arbitrary) selection of the field prime and generator.

RFC 7919 defines a family of finite fields all constructed from the
natural logarithm constant "e", intended to be used as well-known
fields where the peer simply names the field (e.g. "ffdhe2048") rather
than providing the raw prime and generator values.

Add support for this family of finite fields as key exchange
algorithms, to allow for protocols such as TLS version 1.3 where
parameterized fields are not permitted.

We choose to support only up to ffdhe4096, since this is sufficient to
exceed the security strength of our RNG (128 bits).

Support for ffdhe6144 and ffdhe8192 could trivially be added by simply
extending the "euler" constant and adding the relevant FFDHE_GROUP()
declarations.  Doing so would approximately double the space
requirements for both read-only data (from 0.5kB to 1kB) and for
uninitialised data (from 3.5kB to 7kB).

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