util: Fix const cast failures when building with -Werror
    
    When building with -Werror 'make check' fails with many
    errors like:
    
    ../src/tests/cmocka/test_authtok.c: In function ‘test_sss_authtok_password’:
    ../src/tests/cmocka/test_authtok.c:98:48: error: cast discards ‘__attribute__((const))’ qualifier from pointer target type [-Werror=cast-qual]
    
    Make sss_authtok_set() @data argument const, and fix its documentation