Only mark all messages as read on the server if that's supported

This change prevents K-9 Mail from establishing a connection to the
server when all messages are marked as read on a POP3 account.

src/com/fsck/k9/controller/MessagingController.java

@@ -2378,7 +2378,7 @@ public class MessagingController implements Runnable {
             Store remoteStore = account.getRemoteStore();
             remoteFolder = remoteStore.getFolder(folder);
 
-            if (!remoteFolder.exists()) {
+            if (!remoteFolder.exists() || !remoteFolder.isFlagSupported(Flag.SEEN)) {
                 return;
             }
             remoteFolder.open(OpenMode.READ_WRITE);

src/com/fsck/k9/mail/store/Pop3Store.java

@@ -283,7 +283,9 @@ public class Pop3Store extends Store {
         @Override
         public void close() {
             try {
+                if (isOpen()) {
                     executeSimpleCommand("QUIT");
+                }
             } catch (Exception e) {
                 /*
                  * QUIT may fail if the connection is already closed. We don't care. It's just