dbus: Add SELinux support

dbus: Add SELinux support

ConnMan needs to identify application in a secure way when they are
using the Session API. The current D-Bus server implemention supports
two types of LSM, POSIX and SELinux. In order to support SMACK or
TOMOYO the D-Bus code base needs to be patch. This is the initial work
to support at least POSIX and SELinux. Maybe in the future we are able
to support also the other LSMs.

The idea behind gsec is to keep the LSM related code together in one
directory. The API introduces in this patch is not any way final. It
will need some more time figuring out how are able to intregrate this
in a nice way.

The current API introduces g_sec_get_selinux_label() which will return
the SELinux context. The function will issuing a
GetConnectionSELinuxSecurityContext method call.

Note, that this function is not documented in the D-Bus
specification. See for more details the source code dbus/bus/drivers.c
and dbus/bus/selinux.c in the D-Bus reference implementation.