Add N_THREAD_LIMIT to limit the max number of threads (upper limit).

[wargio]

Your checklist for this pull request

• [ ] I've read the guidelines for contributing to this repository
• [ ] I made sure to follow the project's coding style
• [ ] I've documented or updated the documentation of every function and struct this PR changes. If not so I've explained why.
• [ ] I've added tests that prove my fix is effective or that my feature works (if possible)
• [ ] I've updated the rizin book with the relevant information (if needed)

Detailed description

This PR allows to create more threads than (previously) allowed, but also sets an hardcoded upper limit (that can be changed as a meson option)