The test used for determining if a file descriptor
tested with fstat points to a directory is traditionally
done by using the S_IFMT mask[^1][^2], e.g.:
    #define S_ISDIR(m)	(((m) & S_IFMT) == S_IFDIR)
In Rust, this translates to 'm & libc::S_IFMT == libc::S_IFDIR'.
is_stdin_directory() uses 'has!(mode, S_IFDIR)', which is
**not** equivalent and causes non-directory sockets to be
incorrectly recognized as directories. This causes uu-tr
to break when used with all sockets created with socketpair,
including ksh93 pipes[^3]. Below is an example Rust program
demonstrating why the current check is bogus:
  fn main() {
     use nix::sys::socket::{socketpair, SockFlag, AddressFamily, SockType};
     use nix::sys::stat::fstat;
     use libc::{S_IFDIR, S_IFMT, mode_t};
     let (_fd1, fd2) = socketpair(AddressFamily::Unix, SockType::Stream, None, SockFlag::empty()).unwrap();
     let mode = fstat(&fd2).unwrap().st_mode as mode_t;
     if mode & S_IFMT == S_IFDIR {   // Equivalent to S_ISDIR()
         println!("Not reached");    // Not a dir, so unreachable
     }
     if mode & S_IFDIR == S_IFDIR {  // Bogus check for S_IFDIR
         println!("BAD");
     }
  }

- is_stdin_directory(): Fix the regression introduced in 3e4221a
  by replacing the bogus check with one equivalent to S_ISDIR().
- test_tr.rs: Add a regression test for this bug.

[^1]: https://sourceware.org/git/?p=glibc.git;a=blob;f=io/sys/stat.h;h=4bea9e9a#l123
[^2]: https://git.musl-libc.org/cgit/musl/tree/include/sys/stat.h?id=047a1639#n51
[^3]: https://github.com/ksh93/ksh/blob/cc5e0692/src/cmd/ksh93/sh/io.c#L98-L102

Fixes uutils#7658

Also fix cargo clippy lint.
(In case it isn't obvious, I'm fairly new to Rust)