Improve run_process defaults if there's a shell

[A5rocks]

Fixes #3237

If shell=True, then it's possible that things we spawn will themselves spawn things. This is a problem because we only kill the parent process and those don't automatically propagate signals. We can solve this by default by making sure shell processes get their own process group which we subsequently kill.

[codecov]

Codecov Report

❌ Patch coverage is 98.50746% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 99.99486%. Comparing base (0124394) to head (00c1ee8).

Files with missing lines	Patch %	Lines
src/trio/_tests/test_subprocess.py	97.67442%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@                 Coverage Diff                  @@
##                 main       #3273         +/-   ##
====================================================
- Coverage   100.00000%   99.99486%   -0.00514%     
====================================================
  Files             128         128                 
  Lines           19414       19473         +59     
  Branches         1318        1327          +9     
====================================================
+ Hits            19414       19472         +58     
- Partials            0           1          +1     

Files with missing lines	Coverage Δ
src/trio/_subprocess.py	100.00000% <100.00000%> (ø)
src/trio/_tests/test_subprocess.py	99.79079% <97.67442%> (-0.20921%)	⬇️

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