src/bin/pxar.rs: Make pxar extract target optional

In order to improve usablity, the target on archive extraction will be the
current working directory by default.
A different target can be provided via the optional --target <PATH> parameter.

Signed-off-by: Christian Ebner <c.ebner@proxmox.com>

tests/pxar.rs

@@ -27,6 +27,7 @@ fn pxar_create_and_extract() {
     Command::new(exec_path)
         .arg("extract")
         .arg("./tests/archive.pxar")
+        .arg("--target")
         .arg(dest_dir)
         .status()
         .unwrap_or_else(|err| {