This is useful for wiping passwords, as it prevents the compiler from
optimizing out a memset to zero before a free()

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

We need to allocate num_services+2 - one extra space for the new service
and one for NULL.

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/1365

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()