Fix ldirectord failed to start, sockaddr_in6 redefined error.

Perl Socket and Socket6 modules both implement, and export by default,
several identical symbols:

perl -we 'use Socket; use Socket6;'
Subroutine main::pack_sockaddr_in6 redefined at /usr/share/perl/5.14/Exporter.pm line 67.
Subroutine main::unpack_sockaddr_in6 redefined at /usr/share/perl/5.14/Exporter.pm line 67.
Subroutine main::sockaddr_in6 redefined at /usr/share/perl/5.14/Exporter.pm line 67.

Even though this has been the case for a long time already,
somehow this apparently started to confuse ldirectord.

See also: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=675913

Fix this by explicitly requesting from Socket6
only what is actually used.

Closes #365.

Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>