Python: Model shell -c command arguments

[theinfosecguy]

Fixes #21735.

Models command arguments passed with -c to recognized POSIX shell interpreters through os.exec*, os.spawn*, os.posix_spawn*, and argument-vector subprocess APIs.

This detects tainted shell commands while leaving ordinary argument vectors unchanged. The modeling is limited to literal shell executables, an exact -c flag, and literal argument sequences.