Fix cp -f, cp -p, chown -RP, chmod -R -P

[kelp]

Summary

• cp -f: Only unlink destination when it cannot be opened for writing. Writable destinations are overwritten in place via truncate+write, preserving hard links and inodes. Previously, -f unconditionally unlinked the destination, severing hard links even when unnecessary.
• cp -p: Add fchown call to copyFileWithAttributes to preserve source uid/gid on the copy. EPERM is silently ignored for non-root users, matching GNU behavior.
• chown -RP: Use lstat instead of stat at the entry of chownRecursive when -P is active (or when neither -H nor -L is set). This prevents following a cmdline symlink-to-directory and incorrectly recursing into the target.
• chmod -R -P: Skip symlinks encountered during recursive traversal instead of applying chmod through them to the target file. GNU chmod silently skips symlinks during -R.

Test plan

• New unit test: cp: -f should not unlink writable destination (preserves hard links) — creates a hard link to dest, copies with -f, verifies inode preserved
• New unit test: cp: -p should preserve group ownership via chown — copies with -p, verifies gid matches
• New privileged test: chown -RP should not follow cmdline symlink to directory — creates symlink to dir, runs chown -RP with verbose, verifies target dir contents not in output (skipped without fakeroot)
• New unit test: chmod: -R -P should not follow symlinks during traversal — creates symlink to outside file, runs chmod -R -P, verifies outside file mode unchanged
• Full test suite: no regressions (38 pre-existing failures unchanged, 4 new tests added, 3 pass, 1 skipped)