gdbusaddress: Use runstatedir rather than localstatedir

This reworks commit 20e1508e, for two reasons:

• Upstream D-Bus now does the same (although this isn’t yet reflected in the online version of the D-Bus Specification); see https://gitlab.freedesktop.org/dbus/dbus/-/merge_requests/209.
• It allows local-prefix (e.g. jhbuild) builds of GLib to build in a custom prefix while still interacting with system services using the system-wide /run directory. To do so, pass -Druntime_dir=/run to meson configure.

As documented in the NEWS file in https://gitlab.freedesktop.org/dbus/dbus/-/merge_requests/209, it’s only valid to use /run – rather than /var/run – for D-Bus if the two paths are interoperable. i.e. /var/run should be a symlink to /run, and the D-Bus daemon should be configured to put its socket there.

This commit deliberately doesn’t introduce a special system_socket configure option for specifying where the D-Bus system socket lives, as that would only be useful for a distribution which sets runstatedir to something other than /var/run or /run, which seems unlikely. We could add such an option in future, though, if a distribution comes forward with such a requirement.

See discussion on !3095 (comment 1605502).

Signed-off-by: Philip Withnall pwithnall@endlessos.org