[SSSD] [sssd PR#355][opened] localauth plugin: change return code of sss_an2ln

[sumit-bose]

   URL: https://github.com/SSSD/sssd/pull/355
Author: sumit-bose
 Title: #355: localauth plugin: change return code of sss_an2ln
Action: opened

PR body:
"""
It is expected that the an2ln plugin function returns KRB5_LNAME_NOTRANS to
indicate that no mapping can be determined and other an2ln methods can be
tried. Currently SSSD's localauth plugin returns KRB5_PLUGIN_NO_HANDLE
which sould only be used for the userok plugin function.

Resolves https://pagure.io/SSSD/sssd/issue/3459

Additionally I added unit tests for the plugin to this pull request.
"""

To pull the PR as Git branch:
git remote add ghsssd https://github.com/SSSD/sssd
git fetch ghsssd pull/355/head:pr355
git checkout pr355