borg 1.x: add length limit to archivename_validator?

`archivename_validator`: master branch has a length limit there, but 1.4-maint branch doesn't.

maybe it would be good to also have the limit in borg 1.x, to avoid issues when migrating to borg2.

even though the validator has no limit yet in borg 1.x, guess that didn't mean arbitrarily long archive names "just worked".

E.g. when mounting a borg repo, the archive names are used as directory names and on Linux NAME_MAX is 255, so a directory name can't be longer than that.

Update: I noticed that this is a bit more than just the length limit of archive names. IIRC comments length might also be different and also accepted content for names / comments. One has to check the metadata validators borg2 uses currently.