_fuse/mount.py: Monitor the fuse process while waiting for the mount

Description

One pitfall with this change is that it will throw a BUG when fuse encounters a problem while mounting.

However, making it a proper BuildStream error would require much more changes on the sandbox side.

All in all, when we hit this it still means fuse is mis-configured and this should disappear once we are using BuildBox

It is also a major improvement compared to now when it's just hanging.

Fix #1158 (closed)

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