Initialize the thread system early in the cold load.

Initialize the thread system early in the cold load.

Thus with-thread-mutex-lock can be used during initialization of most
packages.  Avoid using the global set-interrupt-enables! binding in
dynamic-wind because it is now called so early.

This anticipates removing without-interrupts from gcfinal.scm and thus
calling with-thread-mutex-lock during make-gc-finalizer, e.g. during
the initialization of the (runtime string) package.