Add cmdlinet::has_option

[smowton]

Previously cmdlinet could parse a whole command-line, but it wasn't possible to verify before using cmdlinet::set that an option was valid. This is useful when a driver program receives options in a format other than a string (e.g. a structured list) and wishes to check that the options would have been valid if supplied conventionally, rather than silently drop invalid args.

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[allredj]

Passed Diffblue compatibility checks (cbmc commit: 3f56a37).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/86632089

[allredj]

Passed Diffblue compatibility checks (cbmc commit: 1225b77).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/86633726