use /run for the default unix socket file

The standard for runtime data, such as socket files, is /var/run or /run
This is slightly better than /tmp for miscellaneous reasons

This would require a more involved process, because we'd like wormholed to support being run as a user conveniently:
- as root: `/run/wormhole.sock`
- as user `/run/user/{user}/wormhole.sock`

This could mean that the CLI should check for both of these location for a system-wide (root) daemon or a user daemon.
This would by default mean that any user could not access any other user's wormhole daemon.
Having the `/run/user/{user}/wormhole.sock` would still facilitate running the daemon as a user for development and testing purposes

Ideally, the `/run/wormhole.sock` could be set to have `wormhole` group and be restricted to users of that group. This would be an improvement to security, but would require the following additional steps in the installer:
- create the `wormhole` group
- ask to add the current user to the `wormhole` group
- ask to add any other users to the `wormhole` group

This is inspired by docker's handling of its socket file

The /run/user/ directory is managed by systemd, which is currently an assumed dependency.