using system groups in svnaccess.conf

subversion need to specify group members in the 'groups'
section of
svnaccess.conf configuration file.
external access control system like LDAP, AD, &etc.
requires to syncronize group
members to svnaccess.conf (for example:
http://thoughtspark.org/node/26/).
subversion must query operating system for group members
directly.
for example: on posix systems from nss (ldap, nis,
/etc/group ...). on windows:
from AD or local groups.

authz_posixgroup_contains_user.patch is a prototype for
posix system (getgrnam).

svnaccess.conf may be like that:

[repos1:/]
%wheel = rw
%members.test.bla-bla-bla = r

'%'-prefix means system group

http://subversion.tigris.org/issues/show_bug.cgi?id=4489