This patch allows extra callback options to be passed to the module
implementing the SSH callback module behaviour.

A module implementing the SSH key callback API is used to customize
the handling of public key. This patch allows extra callback options
to be passed to the module implementing the SSH callback module
behaviour.

The key_cb option has been changed:

  {key_cb, atom()} -> {key_cb, key_cb()}

  Where:
	key_cb() :: atom() | {atom(), list()}

The callback options, if specified, is made available to the callback
module via the options passed to it under the key 'key_cb_private'.

More details and some backgorund is available here[1].

[1]: http://erlang.org/pipermail/erlang-patches/2015-November/004800.html

* maint-18:
  Updated OTP version
  Update release notes
  ssh: vsn.mk updated
  ssh: New test cases for SSH_MSG_KEX_DH_GEX_REQUEST_OLD
  ssh: update existing testcases
  ssh: Improve group selection
  ssh: implemented server side SSH_MSG_KEX_DH_GEX_REQUEST_OLD for putty client

Conflicts:
	lib/ssh/src/ssh_transport.erl
	lib/ssh/test/ssh_protocol_SUITE.erl
	lib/ssh/vsn.mk

Now it chooses the first found if no exact match.