If i'm using RemoteForward or LocalForward with unix-domain sockets, sometimes i want to specify a socket that is relative to the homedir on the remote system.

It would be nice to be able to specify this simply in the appropriate place in ssh_config without having to know and hard-code my homedir for each remote host into host-specific stanzas.

for example, it would be nice to be able to use:

    ssh -o 'RemoteForward=~/.foo:~/.foo' $host

and have them be relative to the respective home directories, even if it's `/home/dkg/.foo` on the local system and `/users/example.org/admin/.foo` on the remote system.

I note that using relative paths for the remote side of forwarding links seems to treat them as "relative" to the root directory.  so:

    ssh -o 'RemoteForward=foo/bar:foo/bar' $host

uses /foo/bar on the remote side, but treats the local side as relative to the cwd of the ssh client (note that this gets confusing when using socket multiplexing, but that is a separate issue).

I also note that it might be nice to have other expansions besides the homedir, which makes me wonder whether it might make more sense to use tokens here (c.f. "TOKENS" section in sshd_config(5), at least %h and %U and %u), so that we could use:

    ssh -o 'RemoteForward=%h/.foo:/home/dkg/.foo' $host

But i don't know how to introduce such a change without breaking some potential existing configurations.  Perhaps if the Unix-domain socket path is prefixed with a !, then the tokens would be applied?

I think this would need to be handled on both the ssh and sshd side to make it work, though, and we'd need to sort out whether to retcon this into the definition of `streamlocal` extension (defined in section 2.4 of PROTOCOL) or to define a new extension.