s3-libads: use dns name to open a ldap session

Required for working certificate verification.

Sorry, but I do not remember the case, which I've tried to fix in 2014. Sambas selftest works fine with the changes, now. This pach seems to solve requested issues, now - so this might be fine, now.

Please add tests or additional patches if required.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=13124 Signed-off-by: Björn Baumbach bb@sernet.de