bug#36915: make-thread-local-fluid ignores the default value

[Caleb Ristvedt]

In Guile 2.2.6 as packaged on Guix System (x86-64), the following
happens:

--------------------------------------
(define some-fluid (make-thread-local-fluid 42))

(display (fluid-ref some-fluid))

=> #f
--------------------------------------

The same problem doesn't happen with non-thread-local fluids. The manual
says pretty clearly "whose initial value is DFLT", so this should be a
bug.

This is especially significant because, if I understand correctly, that
"default" value is the initial value of that fluid in a new thread. So
just using fluid-set! immediately after creating it isn't sufficient to
work around it entirely.

- reepca