Allow stripping cwd prefix with explicit search paths

[pederbe]

Summary

• Allow --strip-cwd-prefix with positional paths and --search-path by removing the argument conflicts and limiting the no-search-path check to auto mode.
• Default, auto, never, and dash-filename protection retain their behavior. Add --strip-cwd-prefix to the issue's command to get fileA and b/c/fileC.
• Updated long help, manual, and changelog. Corrected the manual's inverted description of auto mode.

Fixes #2113

Testing

• Regression exercises both path syntaxes, all modes, print0, mixed roots, and a dash filename. Failed before the fix on the argument conflict, passed after it.
• cargo test --locked: 147 unit tests and 95 integration tests passed on Windows GNU.
• cargo fmt --all --check passed after formatting.
• cargo clippy --locked --all-targets --all-features -- -D warnings failed on two existing dead-code warnings in tests/testenv/mod.rs. With -A dead-code appended, passed.

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