fix: skip special files instead of failing to compress

[jadhavgaurav]

Problem

Compressing a directory that holds a socket or a fifo does not produce an archive.

$ ouch compress app out.tar.zst
[INFO] Compressing "app"
[INFO] Compressing "app/daphne.sock"
[ERROR] unsupported file type socket
 - found at "app/daphne.sock"

Same for .zip, and for .7z it fails later with failed to open file ... Operation not supported on socket (os error 102). tar skips these files and writes the archive, which
is what the issue asks for.

A fifo is worse: Handle::from_path opens the path to compare it with the output file, and
opening a fifo blocks until a writer shows up, so ouch compress dir_with_a_fifo out.tar
hangs forever with no output. I hit this while testing the socket fix.

Change

• read_file_type returns Ok(None) for special files (sockets, fifos, block and character
  devices) instead of an error. Only the two archive builders called it.
• The tar and zip builders skip those entries with a warning, and the 7z builder does the
  same using the metadata it already reads. The check runs before the output-file
  comparison, so nothing opens the path and the fifo case cannot block.

$ ouch compress app out.tar.zst
[INFO] Compressing "app"
[WARNING] Skipping "app/daphne.sock", archives cannot store special files like sockets and fifos
[WARNING] Skipping "app/pipe", archives cannot store special files like sockets and fifos
[INFO] Compressing "app/regular.txt"
[INFO] Successfully compressed to "out.tar.zst"

Regular files, directories and symlinks are untouched.

Tests

New special_files_are_skipped_when_compressing in tests/integration.rs, unix only. It
puts a regular file, a unix socket and a fifo in one directory, compresses it to tar, zip
and 7z, decompresses, and asserts the regular file is the only entry.

Checked against the unmodified sources with the test in place: it fails there (the socket
makes compression exit 1, the fifo hangs until the run is killed). With the change it
passes in under a second.

Local runs on the pinned 1.93.0 toolchain: cargo +nightly fmt -- --check clean,
cargo clippy --all-targets -- -D warnings clean, cargo test green.

Fixes #851