krb5_plugin: Add winbind localauth plugin for MIT Kerberos

krb5_plugin: Add winbind localauth plugin for MIT Kerberos

Applications (like OpenSSH) don't know about users and and
their relationship to Kerberos principals. This plugin allows that
Kerberos principals can be validated against local user accounts.

Administrator@WURST.WORLD -> WURST\Administrator

https://web.mit.edu/kerberos/krb5-latest/doc/plugindev/localauth.html

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13480

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Alexander Bokovoy <ab@samba.org>
Autobuild-User(master): Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date(master): Thu Jun 21 15:52:02 CEST 2018 on sn-devel-144