Don't escape ! in r_str_escape_sh on Unix ##util - #27287

I'd hold off merging this as-is. I reproduced three regressions against cp -f:

• Directory destinations fail with EISDIR. r_file_copy("src", "existing_dir") used to create existing_dir/src. There is still a caller relying on this in libr/util/rvc_rvc.c:file_copyp(). Please keep that behavior and cover it.
• The self-copy check happens after the EACCES fallback has already unlinked the destination. With a mode-0400 source, copying src to ./src returns true and replaces its inode. cp -f refuses and leaves it alone. Check identity before unlinking too. Please add a read-only alias/hardlink case; the current self-copy test uses a writable source.
• Sparse files lose their holes. A 64 MiB sparse source used zero allocated blocks after cp -f, but about 64 MiB after this helper. Large disk images copied through prj.files can now consume much more space and hit ENOSPC. Please preserve holes if we keep the native copy.

These checks used the exact helper in a small Linux harness, with DAC override disabled for the permission case. I haven't run the full testsuite.

Do we need the native-copy rewrite in this fix? Could we first remove the Unix ! escaping from r_str_escape_sh? Noninteractive sh does not need it, and that would also fix the nearby r_file_move fallback with a much smaller patch. The command-length limit and native copying could then be handled separately.

Thanks for testing these. You're right that the native copy isn't needed for this bug, so I replaced it with your suggestion: the PR now only drops the ! escape from r_str_escape_sh on Unix, which also fixes the r_file_move fallback and the other callers that build sh -c commands. The new test copies copy_src!$`"\ through r_file_copy and fails on master; native copying and the command-length limit can be a separate change.