Unwindset parser duplicated

[NathanJPhillips]

See https://github.com/diffblue/cbmc/blob/develop/src/cbmc/bmc.cpp#L562 and https://github.com/diffblue/cbmc/blob/develop/src/goto-instrument/unwind.cpp#L53. The former is better but is specific to BMC, the latter includes comparisons to signed -1 instead of std::numeric_limits::max. As a result another duplicate has been made in https://github.com/diffblue/test-gen/pull/1236 which should be moved somewhere in CBMC and used by the existing two implementations.

[martin-cs]

sigh I'd hoped we'd moved on beyond duplicated functionality (remembering that time when we had four loop detection systems...). Yes; it would be good to not have this repeated. If your suggestion with test-gen is to be implemented (and it does sound like a good idea) it will have to be done by someone with access to test-gen (and the delegated legal authority to contribute it under BSD 4 clause).

[tautschnig]

This was addressed in #2167.