The getXXYYY_r() calls require a buffer to store the variable data of
the passwd and group structs. If the provided buffer is too small ERANGE
is returned and the caller can try with a larger buffer again.
Cmocka/cwrap based unit-tests for get*_r_wrapper() are added.
Resolves https://fedorahosted.org/freeipa/ticket/4908
Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
