LDAP: fix bad free() when URL parsing failed

When an error occurs parsing an LDAP URL, The ludp->lud_attrs[i] entries
could be freed even though they sometimes point to data within an
allocated area.

This change introduces a lud_attrs_dup[] array for the duplicated string
pointers, and it removes the unused lud_exts array.

Bug: http://curl.haxx.se/mail/lib-2013-08/0209.html