guestfish: Don't exit with a failure if guestmount doesn't terminate

If we exit with a failure we can't try to kill the process up to 30
times.
make: Import run_sync

It's needed to run tail -f on the output log.
make: Run tail -f on the new output log location

When the task completes the whole working directory is moved to the
successful/ directory and the path we have previously calculated is not
valid anymore.

Recalculate the path because task._workdir has now been changed to the
new location.
make: Continue after running tail

In single-task-mode, when the task finishes we just show the output tail
and continue.