gio: implement unixexec dbus server address support

unixexec can be used to execute an application that gets connection to the bus. For example unixexec can be used to start systemd-stdio-bridge over ssh to make remote dbus connections. (Please note that fd passing doesn't work for this use case)

For more information about unixexec https://dbus.freedesktop.org/doc/dbus-specification.html

Closes #1275 Original issue thread and patch author https://bugzilla.gnome.org/show_bug.cgi?id=784077

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authored-by: Umut Tezduyar Lindskog umuttl@axis.com