main: Use GBusNameOwnerFlags to specify flags

We are still using DBUS_NAME_FLAG_REPLACE_EXISTING from dbus-glib
in some places (whose headers are drawn in via telepathy-glib).
As both GIO and dbus-glib use the values that will end up being
sent on the bus, the define and enum value are interchangeable,
but it's clearly better to consistently use the library that is
explicitly included.

https://bugzilla.gnome.org/show_bug.cgi?id=771721