Small POP3 improvements

app/src/main/java/eu/faircode/email/Core.java

@@ -1207,6 +1207,9 @@ class Core {
 
             Message[] imessages = ifolder.getMessages();
             Log.i(folder.name + " POP messages=" + imessages.length);
+
+            // Some servers send messages once only
+            // Prevent deleting all local messages
             if (imessages.length == 0)
                 return;
 
@@ -1228,7 +1231,7 @@ class Core {
 
                     MessageHelper helper = new MessageHelper((MimeMessage) imessage);
 
-                    String msgid = null;
+                    String msgid;
                     if (caps.containsKey("UIDL")) {
                         String uid = ifolder.getUID(imessage);
                         if (existing.contains(uid))

app/src/main/java/eu/faircode/email/EntityAccount.java

@@ -85,7 +85,7 @@ public class EntityAccount extends EntityOrder implements Serializable {
     @NonNull
     public Boolean notify = false;
     @NonNull
-    public Boolean browse = true; // Leave messages on server
+    public Boolean browse = true; // POP3: Leave messages on server
     @NonNull
     public Boolean auto_seen = true;
     public Character separator;

app/src/main/java/eu/faircode/email/MailService.java

@@ -242,6 +242,7 @@ public class MailService implements AutoCloseable {
         //System.setProperty("mail.socket.debug", Boolean.toString(debug));
 
         if ("pop3".equals(protocol) || "pop3s".equals(protocol)) {
+            isession.setDebug(true);
             iservice = isession.getStore(protocol);
             iservice.connect(host, port, user, password);