Remove or fix installation of signcontrol

[eagle]

signcontrol is currently unconditionally installed even though it has per-site configuration, so that configuration would be overwritten on make update.

It's not clear that INN should be shipping this program at all. Only a few people will need to issue PGP-signed control messages, and those who do can easily get it as part of the pgpverify distribution (or download one of the many other programs for this purpose).

[Julien ÉLIE]

In order to prevent replay of old control articles, the Injection-Date: header field should be added to @signheaders and @orderheaders (and also be computed as the Date: header field).