Support negative values on nthreads

[virtualritz]

Description

This makes nthreads accept negative numbers to mean m_pool->size() + nthreads. I.e. on a 16 core machine specifying nthreads as -2 would make OIIO use 14 cores.
If m_pool->size() < -nthreads the result is clamped at 1. I.e. a single thread would be used.

On that note: the original resolve() does not have a check for the case where m_maxthreads > m_pool->size() and should possibly be clamped to the latter, if so. I kept it that way. Is that intended/a feature or an oversight?

Tests

Checklist:

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have ensured that the change is tested somewhere in the testsuite
  (adding new test cases if necessary).
• If I added or modified a C++ API call, I have also amended the
  corresponding Python bindings (and if altering ImageBufAlgo functions, also
  exposed the new functionality as oiiotool options).
• My code follows the prevailing code style of this project. If I haven't
  already run clang-format before submitting, I definitely will look at the CI
  test that runs clang-format and fix anything that it highlights as being
  nonconforming.

[linux-foundation-easycla]

• ✅login: virtualritz / (0655bfd)
• ✅login: virtualritz / (0655bfd, 1d7e702)
• ✅login: virtualritz / (0655bfd, 1d7e702, 79f2bf8)

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