Try to set DTR on best effort on POSIX

Deciding on whether the actual device supports setting DTR based on the
baud rate does not play out for Linux pseudo terminals. Let's try on
best effort then and ignore ENOTTY/ErrorKind::Unsupported.

src/posix/error.rs

@@ -32,6 +32,7 @@ impl From<nix::Error> for Error {
             E::EINTR => K::Io(IO::Interrupted),
             E::EACCES => K::Io(IO::PermissionDenied),
             E::ENOENT => K::Io(IO::NotFound),
+            E::ENOTTY => K::Io(IO::Unsupported),
             _ => K::Unknown,
         };
         Error::new(kind, e.desc())

src/posix/tty.rs

@@ -192,11 +192,14 @@ impl TTYPort {
             baud_rate: builder.baud_rate,
         };
 
-        // Ignore setting DTR for pseudo terminals (indicated by baud_rate == 0).
-        if builder.baud_rate > 0 {
-            if let Some(dtr) = builder.dtr_on_open {
-                port.write_data_terminal_ready(dtr)?;
-            }
+        // Try to set DTR on best effort. This operation fails for certain devices like Linux
+        // pseudo terminals but we have currently no strategy for reliably detecting whether this
+        // is supposed to work or not.
+        if let Some(dtr) = builder.dtr_on_open {
+            port.write_data_terminal_ready(dtr).or_else(|e| match e {
+                e if e.kind() == ErrorKind::Io(io::ErrorKind::Unsupported) => Ok(()),
+                e => Err(e),
+            })?;
         }
 
         Ok(port)