Implement a mkfilter_group_bymemberdn() function

This was part of a bigger change to implement nested groups, however most of
the other parts were re-implemented differently.

For the original changes, see:
http://lists.arthurdejong.org/nss-pam-ldapd-users/2013/msg00034.html
Implement support for nested groups in nslcd

This differs from the code provided by Steve Hill in that it avoids
(recursively) performing parallel LDAP searches by queueing groups and check
for extra members per queued group (in the forward lookup) or check for extra
parents (for the user to groups lookup).

For the reverse lookup handling the NSLCD_HANDLE macro could no longer be used
because extra care should be taken to free the sets before returning and two
search phases are needed.
Implement support for nested groups in pynslcd
Implement a nss_nested_groups configuration option

This option can be used in both nslcd and pynslcd to enable recursive group
member lookups. By default the functionality is disabled. This also updates
the documentation.
Add tests for nested group functionality

This also includes some changes to the test directory contents that were for
other tests and functionality.
Implement support for nested groups