Fix setting $DBUS_SESSION_BUS_ADDRESS after Emacs has started

* doc/misc/dbus.texi (Alternative Buses): Explain using
$DBUS_SESSION_BUS_ADDRESS after Emacs has started.

* src/dbusbind.c (XD_DBUS_VALIDATE_BUS_ADDRESS): Use egetenv.

doc/misc/dbus.texi

@@ -1847,8 +1847,24 @@ This function establishes the connection to D-Bus @var{bus}.
 corresponding bus.  For the system and session buses, this function is
 called when loading @file{dbus.el}, there is no need to call it again.
 
-The function returns the number of connections this Emacs session has
-established to the @var{bus} under the same unique name
+If Emacs was invoked when there was no D-Bus session bus available
+yet, you can set the environment variable
+@env{DBUS_SESSION_BUS_ADDRESS} once the session bus daemon is running
+and offering the address.  Calling @code{dbus-init-bus} initializes
+the connection to the session bus.
+
+@lisp
+(setenv "DBUS_SESSION_BUS_ADDRESS" "unix:path=/run/user/1000/bus")
+
+@result{} "unix:path=/run/user/1000/bus"
+
+(dbus-init-bus :session)
+
+@result{} 2
+@end lisp
+
+@code{dbus-init-bus} returns the number of connections this Emacs
+session has established to the @var{bus} under the same unique name
 (@pxref{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

src/dbusbind.c

@@ -276,7 +276,7 @@ XD_OBJECT_TO_STRING (Lisp_Object object)
 
 #define XD_DBUS_VALIDATE_BUS_ADDRESS(bus)				\
   do {									\
-    char const *session_bus_address = getenv ("DBUS_SESSION_BUS_ADDRESS"); \
+    char const *session_bus_address = egetenv ("DBUS_SESSION_BUS_ADDRESS"); \
     if (STRINGP (bus))							\
       {									\
 	DBusAddressEntry **entries;					\