Added framework to sshbn.c to make it possible to vary the

Added framework to sshbn.c to make it possible to vary the
underlying integer type forming the Bignum. Using this, arranged
that gcc/x86 uses 32-bit chunks rather than the guaranteed ANSI-
portable 16-bit chunks. This has gained another 30% on key exchanges
by my measurements, but I'm not yet convinced that it's all
perfectly robust - it seems to work fine for SSH1 and SSH2/RSA but
I haven't ensured that every bignum routine is actually being
tested, so it may yet show up problems in DSA or key generation.

[originally from svn r3135]