Consider storing metadata (UNISONLOCALNAME, ar/fp files) within a replica

[gdt]

Currently, unison stores ar/fp metadata in .unison, and gethostname/UNISONLOCALNAME is implicitly a property of the host. There are two practices for which this is problematic:

1. mounting various external disks on /mnt, as is ancient UNIX tradition.
2. mounting an external disk on multiple machines

For both, having the archives with the data, rather than with the machine, would simplify usage and make it more robust. In case 1, people could run unison from a remote machine to dest:/mnt and have each destination treated separately. In case 2, one could sync to foo:/mnt and later bar:/mnt, considering the disk the target of the sync and which host is currently fronting for it as unimportant.

This approach has the theoretically-pleasing advantage of keeping metadata and data bound to each other. It of course almost certainly has disadvantages.

This is complementary to but separate from #1026 which is about moving from hostname to some stored unique name/id.