[SSSD] [PATCH] start adding an attribute map for generic attributes

[Simo Sorce]

While adding infrastructure to use the USN counter for enumerations I
found that sdap_id_map is not really a good name as the map represent a
generic attribute map. This patch renames the structure and adds some
generic infrastructure for non user/group specific attribute mappings.

This infrastructure is not used yet, will be in the next patches.

Simo.