Reconsider gethostname/UNISONLOCALNAME vs stored name in .unison

[gdt]

Currently unison uses the hostname archive names. This is problematic for hosts without stable names (e.g., those using dhcp without a local overrride to have the hostname as seen by gethostname(3) stable), and the UNISONLOCALNAME alternative appears too difficult in practice.

This ticket is to consider and perhaps implement alternatives, e.g.:

• generate a name from the hostname (with a uniquifier, UUID-ish) and store it in .unison, using that instead
• as above, but stop pretending that it is a name

Also in scope for this issue is a tool to modify archive files to have a new name, given archives with an old name.

See also #1036 which is about keeping this sort of information bound to the replica instead of on the host that (currently) has the replica's filesystem.