Don't throw error for PAM_TEXT_INFO in PAM conversation function

The PAM conversation function currently throws an error if it encounters any messages that are not PAM_PROMPT_ECHO_OFF or PAM_PROMPT_ECHO_ON:

https://github.com/mariadb-corporation/MaxScale/blob/maxscale-2.3.6/server/modules/authenticator/PAM/PAMAuth/pam_client_session.cc#L117

This means that a user's MaxScale log can fill up with error messages like the following:

2019-05-05 19:40:42   error  : Unexpected PAM message: type='4', contents='Your password will expire in 1 day(s).'

If msg_style=4, then it is of the type PAM_TEXT_INFO:

$ grep "PAM_TEXT_INFO" /usr/include/security/_pam_types.h

#define PAM_TEXT_INFO           4

These appear to be harmless messages:

PAM_TEXT_INFO
Display some text.

http://www.linux-pam.org/Linux-PAM-html/mwg-expected-by-module-item.html#mwg-pam_conv

That doesn't really seem worthy of an error that causes the connection to fail.

It seems like it would be better to log PAM_TEXT_INFO messages with MXS_NOTICE or MXS_INFO instead of MXS_ERROR, and then also return PAM_SUCCESS, so it doesn't count as a failure.