ssh, sshd: Add the VersionAddendum configuration variable

[emaste]

Summary

Original commits:
db58a8e40ce0ed8b27d51fd274ab609b18d74273 by des
c6de6086cfc759413dd95b5fa549fe7eebca801a by emaste

[jlduran]

I guess, the man pages also need their default version string updated?

[allanjude]

kex.c

1192

is this not redundant now? as above on line 1188, if *version_addendum == '\0' it sets version_addendum = NULL;

ssh.c

891

ssh_config

47

Does this date need bumping?

ssh_config.5

1882

We should sync this default

sshd.c

903

[emaste]

Updates from @allanjude

[emaste]

I guess, the man pages also need their default version string updated?

Yes, all instances should be updated at commit time. I've used 2021XXXX as a placeholder for now.

[emaste]

rebase to accommodate upstream changes

[emaste]

style from @allanjude

[emaste]

kex.c

1192

Yes, I think this came out of upstream implementing this differently, reverting locally.

[emaste]

Incorporate more feedback from @allanjude

[emaste]

This is still in the tree today, but propose retiring it in D32930