Neither libssh2_userauth_password_ex() nor
libssh2_userauth_keyboard_interactive_ex() would return a login failure
error if the server responded with a SSH_MSG_USERAUTH_FAILURE, instead
you would see whatever previous error had occurred, typically
LIBSSH2_ERROR_EAGAIN.
This patch changes error code -18 to LIBSSH2_ERROR_AUTHENTICATION_FAILED
and makes LIBSSH2_ERROR_PUBLICKEY_UNRECOGNIZED an alias for
LIBSSH2_ERROR_AUTHENTICATION_FAILED. In addition, new logic in
userauth_password() properly handles SSH_MSG_USERAUTH_FAILURE and both
this function and userauth_keyboard_interactive() now properly return
LIBSSH2_ERROR_AUTHENTICATION_FAILED.
