[kex.c kex.h kexdh.c kexdhc.c kexdhs.c kexgex.c kexgexc.c kexgexs.c] remove hardcoded hash lengths in key exchange code, allowing implementation of KEX methods with different hashes (e.g. SHA-256); ok markus@ dtucker@ stevesk@
[kex.c kexdh.c kexgex.c kex.h sshconnect2.c sshd.c ssh-keyscan.c] split kex into client and server code, no need to link server code into the client; ok provos@