Add a simple example program that uses pivot_root. (#1117)

Add an example program that uses `pivot_root`, similar to the
common shell utility.

Author: sunfishcode

examples/pivot_root.rs

@@ -0,0 +1,27 @@
+//! A wrapper around `rustix::fs::pivot_root`.
+
+#[cfg(all(target_os = "linux", feature = "fs", feature = "process"))]
+fn main() -> rustix::io::Result<()> {
+    let mut args = std::env::args();
+    if args.len() != 3 {
+        eprintln!("Usage: {} new_root put_old", args.next().unwrap());
+        std::process::exit(1);
+    }
+
+    let _argv0 = args.next().unwrap();
+    let new_root = args.next().unwrap();
+    let put_old = args.next().unwrap();
+
+    rustix::process::pivot_root(new_root, put_old)?;
+
+    Ok(())
+}
+
+#[cfg(any(
+    not(target_os = "linux"),
+    not(feature = "fs"),
+    not(feature = "process")
+))]
+fn main() -> Result<(), &'static str> {
+    Err("This example requires --features=fs,process and is only supported on Linux.")
+}