Truncate --log file on open instead of appending

Each scpcap invocation now starts its log fresh. Appending meant a re-run
after a failed attempt mixed new output in with (or could be mistaken for)
stale output from the previous run -- confusing when diagnosing exactly
this kind of issue in production.

src/tracelog.rs

@@ -15,8 +15,11 @@ impl Logger {
         Logger(None)
     }
 
+    /// Truncates any existing file at `path` -- each run starts a fresh
+    /// log, so a re-run never leaves stale lines from a previous attempt
+    /// mixed in with (or mistaken for) the current one.
     pub fn open(path: &Path) -> std::io::Result<Logger> {
-        let f = OpenOptions::new().create(true).append(true).open(path)?;
+        let f = OpenOptions::new().create(true).write(true).truncate(true).open(path)?;
         Ok(Logger(Some(Mutex::new(f))))
     }