tape: add pmt command line tool

Experimental, not installed by now.

src/tape/drive/linux_tape.rs

@@ -193,7 +193,7 @@ impl LinuxTapeHandle {
         Ok(())
     }
 
-    fn mtload(&mut self) -> Result<(), Error> {
+    pub fn mtload(&mut self) -> Result<(), Error> {
 
         let cmd = mtop { mt_op: MTCmd::MTLOAD, mt_count: 1, };