[SSSD] [PATCH] Add Winbind provider.

This patch adds the whole Winbind provider. We agreed with Summit, that it would be better to submit as a single patch as splitting it wouldn't make review any easier. It has the ID and AUTH providers. ID can do user, groups, initgroups and enumeration of them. AUTH can do authentication and password change operation. To build the provider, you need to use the --enable-experimental-winbind-provider configure flag. samba-winbind-devel package is required. SSSD configuration option for the Winbind provider can be found in /etc/sssd/sssd.api.d/sssd-winbind.conf. The correspond pretty much to Winbind options normally found in smb.conf. Both NT and AD domains have to be joined first using samba net utility: net ads join -S server -U user ... A man page (man sssd-winbind) is coming soon. Big thanks to Summit for helping me out with testing this a reviewing commit in my private repo periodically. Thanks! Pavel