Added new parameter to split_on_separator that allows to skip
empty values.

The whole function was rewritten. Unit test case was added to
check the new implementation.

https://fedorahosted.org/sssd/ticket/1484

string_in_list() and add_string_to_list() are two utilities for NULL
terminated strings arrays. add_string_to_list() adds a new string to an
existing list or creates a new one with the strings as only item if
there is not list. string_in_list() checks if a given string is in the
list. It can be used case sensitive or in-sensitive.

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

We'll be using it on various places of the SSSD. The function is in its
own file to allow using just the one piece without having to drag in the
whole util.c module.

Also adds a unit test for sss_atomic_io()

This test always generate a random string so each time the test is run we will
test the hash function with a new value.
It also hashes the same string twice and compares the result so that we have a
chance of catching if uninitialized variables are getting mixed into the value
calculation and end up generating different results for the same input.

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);