install: implement -C / --compare (#1811)

* install: implement `-C` / `--compare`

GNU coreutils [1] checks the following: whether
- either file is nonexistent,
- there's a sticky bit or set[ug]id bit in play,
- either file isn't a regular file,
- the sizes of both files mismatch,
- the destination file's owner differs from intended, or
- the contents of both files mismatch.

[1] https://git.savannah.gnu.org/cgit/coreutils.git/tree/src/install.c?h=v8.32#n174

* Add test: non-regular files

* Forgot a #[test]

* Give up on non-regular file test

* `cargo fmt` install.rs

Cargo.toml

@@ -334,6 +334,7 @@ conv = "0.3"
 filetime = "0.2"
 glob = "0.3.0"
 libc = "0.2"
+nix = "0.20.0"
 rand = "0.7"
 regex = "1.0"
 sha1 = { version="0.6", features=["std"] }