* dbus/dbus-server.c (dbus_server_listen): allow abstract sockets
	using unix:abstract=/foo, and when listening in a tmpdir
	i.e. unix:tmpdir=/tmp, always use abstract sockets if we can.

	* dbus/dbus-transport.c (_dbus_transport_open): support
	unix:abstract=/foo

	* dbus/dbus-server-unix.c (_dbus_server_new_for_domain_socket):
	support abstract sockets

	* dbus/dbus-transport-unix.c
	(_dbus_transport_new_for_domain_socket): support abstract sockets

	* dbus/dbus-sysdeps.c (_dbus_connect_unix_socket): add "abstract"
	toggle as an argument, implement abstract namespace support
	(_dbus_listen_unix_socket): ditto

	* configure.in: add --enable-abstract-sockets and implement
	a configure check for autodetection of the right value.