from_thread.run(_sync)? does not set sniffio on asyncio

[gschaffner]

note that from_thread.run(_sync)? does set sniffio on trio, however.

this causes a few bugs:

1. from_thread.run(_sync)? fails when used to call sniffio-dependent functions when the to_thread worker is running a non-asyncio async backend known to sniffio (i.e. trio, curio, or (situationally) trio-asyncio) and the thread that started the worker is on asyncio.

   reproducer: bc3cc49.

2. because BlockingPortal._spawn_task_from_thread shares its underlying implementation with from_thread.run_sync on asyncio, using a BlockingPortal that was started by start_blocking_portal("asyncio") in a non-asyncio async backend known to sniffio deadlocks.

   reproducer: cd9deb1.

[tapetersen]

At least for sync callbacks in asyncio this is an issue I also ran into and wanted to solve by detecting asyncio as backend if we were running a sync task directly in sniffIO. Would be interested in either this solution or mine going forward

issue python-trio/sniffio#35
PR python-trio/sniffio#36

[gschaffner]

@tapetersen i think python-trio/sniffio#36 would still not fix this in all cases. even if sniffio properly detected asyncio callbacks that aren't part of a task, sniffio won't run that detection code if sniffio.thread_local or sniffio.current_async_library_cvar are set. and an incorrect sniffio.current_async_library_cvar can be inherited from the code that calls from_thread.run(_sync)?!

(linking #501 for convenience—#524 would fix it!)