s3/ldap: delay the ldap search alarm termination a bit

s3/ldap: delay the ldap search alarm termination a bit

do the alarm termination of the the ldap search a bit delayed so the LDAP
server has a chance to tell us that the time limit was reached and the
search was abandoned. If the search is terminated this way we also get
the correct LDAP return code in the logs. If alarm() stops the search the ldap
search routine will report that the LDAP server is down which would trigger us
to rebind to the server needlessly which we also want to avoid.