Append *default-special-bindings* set from elsewhere on thread creation

[leetwinski]

Follow up on this discussion in stmx repository.
We found out that *default-special-bindings* special variable gets overwritten by kernel bindings in with-thread macro on thread creation.

The patch solves this, appending the possibly existing values to kernel bindings. Fixes the interoperability issue with stmx and possibly other libraries, relying on those thread bindings.

[cosmos72]

I am stmx author, and I confirm that removing some (or all) existing conses (symbol . form) from bordeaux-threads:*default-special-bindings*, as lparallel's (with-thread) macro in src/thread-util.lisp does,

breaks any library (including mine) that add conses to it and rely on them to remain present.

This patch looks correct to me: many libraries have reasons to add conses to bordeaux-threads:*default-special-bindings*, including at least both stmx and lparallel. But I see no reasonable use case where a library should remove conses added by someone else.

Also, bordeaux-threads docs explicitly state

The effect of mutating this list is undefined, but earlier forms take precedence over later forms
for the same symbol, so defaults may be overridden by consing to the head of the list.

so even in case a symbol already appears bordeaux-threads:*default-special-bindings*, the correct approach for overriding it is to prepend another cons containing a different value for it.

[sabracrolleton]

Does anyone have any problems with this pull request?

[cosmos72]

As it can (hopefully) be deduced from my post above, I am definitely in favor of merging this pull request.

[leetwinski]

I couldn't locate anyone able to approve and/or merge this. Also tried to reference it in sharplispers/lparallel. Also no luck.

[mdbergmann]

Last time I had luck make people aware in Reddit.