Put framework for new control flow in place.

As of this commit, only regular syscalls use the new flow.  The other
execution steps will require significant extra work to use the changed
flow.  However, this puts enough in place to finish breakpoints, and
hopefully they'll "work" most of the time with only regular syscalls
aware of them.
Merge pull request #86 from cgjones/refactor-regular-syscall-replay

Put framework for new control flow in place.