grep: support GREP_COLORS mt and warn on deprecated GREP_COLOR

[sylvestre]

No description provided.

[github-actions]

GNU grep testsuite comparison:

Test results comparison:
  Current:   TOTAL: 128 / PASSED: 74 / FAILED: 33 / SKIPPED: 21
  Reference: TOTAL: 128 / PASSED: 73 / FAILED: 34 / SKIPPED: 21

Changes from main branch:
  TOTAL: +0
  PASSED: +1
  FAILED: -1

Test improvements (1):
  + color-colors

[codspeed-hq]

Merging this PR will not alter performance

✅ 10 untouched benchmarks
⏩ 17 skipped benchmarks¹

---

_{Comparing sylvestre:color2 (e24538e) with main (670669e)}

Footnotes

1. 17 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[github-actions]

GNU grep testsuite comparison:

Test results comparison:
  Current:   TOTAL: 128 / PASSED: 74 / FAILED: 33 / SKIPPED: 21
  Reference: TOTAL: 128 / PASSED: 73 / FAILED: 34 / SKIPPED: 21

Changes from main branch:
  TOTAL: +0
  PASSED: +1
  FAILED: -1

Test improvements (1):
  + color-colors