SELinux support from Matthew Rickard <mjricka@epoch.ncsc.mil>

	* bus/selinux.c, bus/selinux.h: new file encapsulating selinux
	functionality

	* configure.in: add --enable-selinux

	* bus/policy.c (bus_policy_merge): add FIXME to a comment

	* bus/main.c (main): initialize and shut down selinux

	* bus/connection.c: store SELinux ID on each connection, to avoid
	repeated getting of the string context and converting it into
	an ID

	* bus/bus.c (bus_context_get_policy): new accessor, though it
	isn't used
	(bus_context_check_security_policy): check whether the security
	context of sender connection can send to the security context of
	recipient connection

	* bus/config-parser.c: add parsing for <selinux> and <associate>

	* dbus/dbus-transport.c (_dbus_transport_get_unix_fd): to
	implement dbus_connection_get_unix_fd()

	* dbus/dbus-connection.c (dbus_connection_get_unix_fd): new
	function, used by the selinux stuff