Add support for _daemon_ as a valid system-username in snapcraft.yaml

[Graham Morrison]

With the release of snapd 2.61, a system-username of `_daemon_` is now supported and preferred over `snap_daemon`.

https://github.com/snapcore/snapd/pull/13052

A value of `_daemon_` cannot currently be parsed in snapcraft (7.5.3.post117+git2d596621) without using `passthrough:`

For example, the following:

  system-usernames:
    _daemon_: shared

produces the following error:

Issues while validating snapcraft.yaml: The 'system-usernames' property does not match the required schema: '_daemon_' is not a valid system-username.