Fix use of unsafe string functions in LS2

:Release Notes:
Replace unsafe string comparison function with analogue function which
handles NULL arguments appropriately.

:Detailed Notes:
String comparison function strcmp does not check passed arguments for
NULL values.
All occurences of strcmp call replaced with call to g_strcmp0 except
cases when passed arguments checked for valid values in code before
function call

:Testing Performed:
Unit tests on desktop build (make check, make test).

:QA Notes:

:Issues Addressed:
[BHV-71] Fix: LS2 uses unsafe string functions

Open-webOS-DCO-1.0-Signed-off-by: Oleksii Rudenko <oleksii.rudenko@lge.com>

Change-Id: I70c6483b943c6127d53a9e1650e9777bb723fdf2
Reviewed-on: https://g2g.palm.com/5098
Reviewed-by: DCO Verification
Tested-by: Oleksii Rudenko <oleksii.rudenko@lge.com>
Reviewed-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>
Reviewed-by: Nikolay Orliuk <nikolay.orliuk@lge.com>