Disallow remote addition of FIDO/PKCS11 provider libraries to

Disallow remote addition of FIDO/PKCS11 provider libraries to
ssh-agent by default.

The old behaviour of allowing remote clients from loading providers
can be restored using `ssh-agent -O allow-remote-pkcs11`.

Detection of local/remote clients requires a ssh(1) that supports
the `session-bind@openssh.com` extension. Forwarding access to a
ssh-agent socket using non-OpenSSH tools may circumvent this control.

ok markus@