Use the new authtok abstraction and interfaces throught the code.

https://fedorahosted.org/sssd/ticket/1495

Removed:
    SSS_UNRESOLVED_DEBUG_LEVEL (completely replaced with SSSDBG_UNRESOLVED)

Added new macro:
    CONVERT_AND_SET_DEBUG_LEVEL(new_value)

Changes unresolved debug level value (SSSDBG_UNRESOLVED) from -1 to 0
so DEBUG macro could be reduced by one condition. Anyway, it has a minor
effect, every time you want to load debug_level from command line parameters,
you have to use following pattern:

    /* Set debug level to invalid value so we can deside if -d 0 was used. */
    debug_level = SSSDBG_INVALID;

    pc = poptGetContext(argv[0], argc, argv, long_options, 0);
    while((opt = poptGetNextOpt(pc)) != -1) { ... }

    CONVERT_AND_SET_DEBUG_LEVEL(debug_level);

https://fedorahosted.org/sssd/ticket/925

Conversion of the old debug_level format to the new one.
(only where it was necessary)

Removed:
  SSS_DEFAULT_DEBUG_LEVEL (completely replaced with SSSDBG_DEFAULT)

proxy.c was growing too large to manage (and some graphical
development tools could no longer open it because of memory
limitations).

This patch splits proxy.c into the following files:
proxy_init.c:   Setup routines for the plugin
proxy_id.c:     Functions to handle user and group lookups
proxy_auth.c:   Functions to handle PAM interactions
proxy_common.c: Common utility routines

We were not passing the old authtok to the pam_chauthtok()
function, causing it to return PAM_AUTH_ERR.

This patch adds a new tevent_req to the proxy provider, which will
spawn short-lived child processes to handle PAM requests. These
processes then call the proxied PAM stack and return the results
via SBUS method reply. Once it is returned, the parent process
kills the child.

There is a maximum of ten child processes running simultaneously,
after which requests will be queued for sending once a child slot
frees up. The maximum processes will be made configurable at a
later date (as this would violate string freeze).