Improved the key exchange API

Improved the key exchange API

crypto_kex_ctx is now differentiated into a client specific context, and
a server specific context.  The distinction is entirely artificial (it's
the same thing under the hood), but it prevents some misuses at compile
time, making the API easier to use.

The name of the arguments have also been changed: "local" and "remote"
have been replaced by "client" and "server" whenever appropriate.  The
previous names made implementation easier, but their meaning was context
dependent, and thus confusing. The new names have stable meanings, and
thus easier to document and use.

TODO: update the manual to reflect those changes.