Establish the connection to D-Bus BUS.
BUS can be either the symbol `:system' or the symbol `:session', or it
can be a string denoting the address of the corresponding bus. For
the system and session buses, this function is called when loading
`dbus.el', there is no need to call it again.
The function returns a number, which counts the connections this Emacs
session has established to the BUS under the same unique name (see
`dbus-get-unique-name'). It depends on the libraries Emacs is linked
with, and on the environment Emacs is running. For example, if Emacs
is linked with the gtk toolkit, and it runs in a GTK-aware environment
like Gnome, another connection might already be established.
When PRIVATE is non-nil, a new connection is established instead of
reusing an existing one. It results in a new unique name at the bus.
This can be used, if it is necessary to distinguish from another
connection used in the same Emacs process, like the one established by
GTK+. It should be used with care for at least the `:system' and
`:session' buses, because other Emacs Lisp packages might already use this connection to those buses. (fn BUS &optional PRIVATE)