Synchronize nsswitch backend code with freeIPA
FreeIPA ipa-extdom-extop plugin uses the same logic as slapi-nis
schema-compat plugin to handle requests to SSSD. Thus, we keep the
code synchronized across both code bases. Since both plugins are loaded
into the same address space we currently rename functions to allow them
to co-exist. In future we'd move some of common code to a shared
library.