RFE: add ability to exclude roles on random generation

[SuperStucco]

Kinda-sorta a placeholder so I don't forget while working on something else, but if someone else gets to it first...

The current random generation system allows the use of 'mission roles' to selectively control what gets generated. For example, the CAVALRY role promotes generation of fast, well armed units while excluding slower ones. What it doesn't have, is an option to also exclude based on additional roles. For example, requesting URBAN while also excluding all FIELD_GUN to generate conventional infantry suitable for urban combat while also excluding those flagged as having field guns.

This would be added through the random generation parameters system, in parallel and similar to how the current force generator roles are being handled. Thankfully the implementation will be much simpler, pulling out table entries that match the exclusion list after they are generated by the existing inclusion list.

This doesn't need to be added to the RAT table generator UI tab under the Create a Random Army dialog, as it's more of a programmers utility. But if someone gets industrious...