Add an "interruptible" option to TBScriptLock(). When set, each time
through the loop we look to see if signals are pending, and if so we return early with an error. The caller (libvnode_xen) can use this to avoid really long waits, when the server has said to stop what its doing. For example, a vnode setup is waiting for an image lock, but the server comes along ands to stop setting up. Previously, we would wait for the lock, now we return early. This is to help with cancelation where it is nice if the server can stop a CreateSliver() in its tracks, when it is safe to do so.
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