client/connection: Add support for the socket existing in /run/incus

[Conan-Kudo]

Transient sockets are supposed to be in /run rather than /var, so make it possible to detect that automatically when used.

[stgraber]

That won't work as /run/incus exists everywhere but most systems use /var/lib/incus/unix.socket

[stgraber]

So the check would need to be about the exact socket path rather than just checking for the directory.

[derkuci]

It seems that putting the socket in /run/incus would require more changes than just this patch. For example, the pre-start hook only looks at /var/lib/incus (its command line is something like /proc/1078/exe callhook /var/lib/incus "default" "vm01" start) and it fails when the socket is in /run/incus.

[stgraber]

Hmm, the server side is supposed to be handled through the INCUS_SOCKET environment variable being passed to incusd, the callhook logic does check for that env variable so it should work fine so long as what calls it doesn't strip the env variable somehow.

[derkuci]

I see. I'll report to Fedora that their RPM spec may miss an environment variable setting (INCUS_SOCKET=/run/incus/unix.socket).