(ITS#6758) Cleaning up SlapReply usage

[h.b.furuseth@usit.uio.no]

Full_Name: Hallvard B Furuseth
Version: 2.4.23, HEAD
OS: Linux x86_64
URL: 
Submission from: (NULL) (129.240.6.233)
Submitted by: hallvard


Slapd and its modules put varous info in SlapReply and send it.
They do not always clean it up afterwards, or not correctly.  Then
they sometimes reuse the SlapReply without reinitializing, even
though most of its fields are invalid at that point.  (In practice
they are usually mostly zeroed by then, so slapd works anyway.)

This is a dup of the misnamed ITS#5340, so the CHANGES file can
refer to an ITS with a better name for all the upcoming commits.

Also I'll go about it a bit backwards than discussed there:
First reset some flags and avoid reusing SlapReply, then get
more aggressive about obeying the flag settings that remain.