ssh-libssh: reduce excessive verbose output about pubkey auth

ssh-libssh: reduce excessive verbose output about pubkey auth

The verbose message "Authentication using SSH public key file" was
printed each time the ssh_userauth_publickey_auto() was called, which
meant each time a packet was transferred over network because the API
operates in non-blocking mode.

This patch makes sure that the verbose message is printed just once
(when the authentication state is entered by the SSH state machine).