Fix ldap-search when WITHDN is non-nil (bug#64089)

Fix ldap-search when WITHDN is non-nil (bug#64089)

* lisp/net/ldap.el (ldap-search): Apply ldap-decode-attribute only to
attribute pairs.
(ldap-search-internal): Parse dn with the same regexp as other attrs;
use its value instead of the whole line.  Include entry into result
only if valid dn is present, to avoid treating process sentinel
messages like an entry.  Fix accidental match data clobbering.
Document return value format.