htex interchange exits badly when custom signal handlers are inherited from user workflow process

[benclifford]

Describe the bug
If a user has installed custom unix signal handlers, the interchange might not exit properly at parsl shutdown (or other strange stuff might happen)

htex interchange shutdown happens via a SIGTERM sent to the interchange.

the interchange process inherits the signal handlers of its parent process (the main workflow process) because it is a multiprocessing fork process. this is probably the wrong thing to happen.

these signal handlers are not necessarily going to shut down the interchange, and may do other non-htex stuff.

debugged this with @yadu and kevin working on funcx

To Reproduce
install a signal handler that does not exit, in your workflow process before initialising parsl.
observe that the interchange process remains after the workflow process ends.

Expected behavior
the interchange should terminate when the main parsl process wants it to terminate

Environment
my laptop. mostly observed with funcx endpoint being the workflow process.

[benclifford]

this should be fixed by #2629