IPv6 detection

["Kurt D. Zeilenga" <Kurt@OpenLDAP.org>]

Thanks to Stig, slapd has support for LDAP over TCP/IPv6
and hopefully the client libraries will be likewise extended.

At present, the code is conditional on LDAP_INET6 being
defined and this must be set by the user (currently via
CPPFLAGS, suggested through --enable-ipv6).  However,
I believe that if the platform supports IPv6, OpenLDAP
should take advantage of this support automatically.
I see no reason to disable IPv6 functionality where
IPv6 is supported.

I'd like Stig to make s/LDAP_INET6/AF_INET6/ changes.
Comments?

Kurt