MAKE-THREAD: add INTERNAL-SERVER option.

When INTERNAL-SERVER is true, this indicates that the process is an "internal
server", which means that it responds to requests for work from other
processes. The main effect of this is that if the only processes that remain
are "internal servers", nothing is going to happen, so lisp quits. In most
implementations this option won't have any effect.

This option currently works only in LispWorks.