Support access to Kerberos KDC over UNIX domain socket

[abbra]

MIT Kerberos PR krb5/krb5#1359 adds support to run KDC locally and only accessible over a UNIX domain socket. SSSD krb5 auth provider cannot currently accept UNIX domain socket path as an address of KDC.

You can use COPR asn/localkdc to test SSSD changes against. This COPR repository provides custom MIT Kerberos build (as well as Samba build to support IAKERB to work with local KDCs but this is unrelated to UNIX domain socket support) and localkdc package that allows easy setup of the local KDC. The latter currently requires SELinux permissive mode (just a warning, as we are working on the policy extensions).