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>