Remove unused parameter from sss_selinux_extract_user

Move SELinux processing to provider. The SELinux processing was distributed between provider and pam responder which resulted in hard to maintain code. This patch moves the logic to provider. IT ALSO REQUIRES CHANGE IN THE SELINUX POLICY, because the provider also writes the content of selinux login file to disk (which was done by responder before). https://fedorahosted.org/sssd/ticket/1743

Add domain to sysdb_search_user_by_name() Also remove unused sysdb_search_domuser_by_name()

Modify priority evaluation in SELinux user maps The functionality now is following: When rule is being matched, its priority is determined as a combination of user and host specificity (host taking preference). After the rule is matched in provider, only its host priority is stored in sysdb for later usage. When rules are matched in the responder, their user priority is determined. After that their host priority is retrieved directly from sysdb and sum of both priorities is user to determine whether to use that rule or not. If more rules have the same priority, the order given in IPA config is used. https://fedorahosted.org/sssd/ticket/1360 https://fedorahosted.org/sssd/ticket/1395

Added some SELinux-related utility functions