Syscallbuf desched scheme does not work if desched is uninterruptible

When running on a network or FUSE file system, many operations such as read/write will result in uninterruptible disk sleep. As a result, our syscallbuf desched mechanism does not work, because the desched signal cannot interrupt the system call. This is observable as either performance degradation or a hang (e.g. if the FUSE server is in the same rr session). I'm thinking it might be possible to have the desched signal be delivered to some other task instead, either the tracer (with some carefulness to make sure they're not lost around calls to waitpid) or some dummy ptraced task (that would then cause waitpid to exit in the relevant cases). Thoughts?

The issue names no files, tests, or entry points. Start by reproducing the syscallbuf desched failure on a network or FUSE filesystem and trace how the desched signal and waitpid interact; done means uninterruptible operations no longer cause the reported performance degradation or hang.