Bug 41549: Correctly handle tor's stdout

Subprocess.jsm manages stdout through a pipe, which we never emptied. As a result, it tor filled it completely, a lot of other stuff stalled.

Thanks to a cypherpunk for providing this change!

After merging it to 12.5, we should backport it to stable.

Closes #41549 (closed).

Partially fixes #41548 (closed) and #41532 (closed).

To test:

1. run something like for i in $(seq 2000); do echo $i; curl 127.0.0.1:9150; done: without the patch it stopped at 712 on my Linux machine, with the patch it reached 2000 as expected
2. keep Ctrl+R on about:tor: without the patch, at a certain point about:tor will stay blank, with the patch it will always load (I've kept Ctrl+R pressed for more than 2 minutes)