_dbus_change_to_daemon_user moved into selinux.c for the --with-selinux
(and audit) case because that's where all of the relevant libcap headers
were being used.  However in the --disable-selinux case this didn't
compile and wasn't very clean.

If we don't have libaudit, use the legacy direct setgid/setuid bits
we had before in dbus-sysdeps-util-unix.c.