Issues with rclone on cygwin

[alexandru-bagu]

I know that Cygwin is not a priority nor even directly supported however while testing borg 2.0.0b14 I ran into an issue which would cause python to hang.
Python version used is 3.9, latest available in Cygwin.

The culprit seems to be a daemon thread spawned to prevent the rclone process from hanging due to the stderr pipe being filled.
I will propose a PR that seems to fix my issues.

[ThomasWaldmann]

@ncw ^^^

[alexandru-bagu]

PR #92