Specify that group-exchange is unsupported for server, and don't use it as a default

[roy-trgscreen]

When running as server, and having a group-exchange kex in your list, kex negotiation can fail even if there is a compatible one, as the server will advertise supporting diffie-hellman-group-exchange-sha256, but then after it was chosen authentication will fail, even if there are lower priority kex options that would work (as group-exchange is not implemented for the server:

ssh2/lib/protocol/kex.js

Lines 1439 to 1446 in a3a39d7

	// TODO: allow user implementation to provide safe prime and
	// generator on demand to support group exchange on server side
	return doFatalError(
	this._protocol,
	'Group exchange not implemented for server',
	'handshake',
	DISCONNECT_REASON.KEY_EXCHANGE_FAILED
	);

#1158).

Clarify that certain algorithms are not supported when running as server, and also not choose them by default.

I would potentially even go as far as to filter out user-specified -group-exhange- options so users don't fall into the same trap as I did.