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
