nss: rewrite nss responder so it uses cache_req Given the size of the current nss responder it was quite impossible to simply switch into using the cache_req interface, especially because most of the code was duplication of cache lookups. This patch completely rewrites the responder from scratch. The amount of code was reduced to less than a half lines of code with no code duplication, better documentation and better maintainability and readability. All functionality should be intact. *Code organization* All protocol (parsing input message and send a reply) is placed in nss_protocol.c. Functions that deals with creating a reply packet are placed into their specific nss_protocol_$object.c files. All supported commands are placed into nss_cmd.c. Functions that deals with cache req are in nss_get_object.c and nss_enum.c. *Code flow for non-enumeration* An nss_getby_$input-type is called for each non-enumeration command. This function parses the input message, creates a cache_req_data structure and issues nss_get_object that calls cache_req. When this request is done nss_getby_done make sure a reply is sent to the client. *Comments on enumeration* I made some effort to make sure enumeration shares the same code for users, groups, services and netgroups. Netgroups now uses nss negative cache instead of implementing its own. Resolves: https://fedorahosted.org/sssd/ticket/3151 Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>