Implement method to call libssh2_userauth_banner

[ScottCUSA]

Hello,

Would it be possible to implement a userauth_banner method to return the userauth banner message from libssh2_userauth_banner?

Maybe return an Option<&str> ?

If I'm reading the libssh2 docs correctly, libssh2_userauth_banner will return NULL until after libssh2_userauth_list is called, assuming the remote system sends a SSH_MSG_USERAUTH_BANNER message.

V/R
Scott