ftpserver.pl: Reworked unrecognised command responses

As the IMAP regex could fail and $1 would not contain the command id
updated the unrecognised command response to be more generic and
realistic (like those used in the command handlers).

Additionally updated the POP3, SMTP and FTP responses.

tests/ftpserver.pl

@@ -3062,7 +3062,7 @@ while(1) {
                 $FTPARG="";
             }
             else {
-                sendcontrol "$1 '$full': command not understood.\r\n";
+                sendcontrol "$full BAD Command\r\n";
                 last;
             }
         }
@@ -3073,7 +3073,7 @@ while(1) {
         elsif($proto eq "pop3") {
             # POP3 long "commands" are base64 authentication data
             unless($full =~ /^[A-Z0-9+\/]*={0,2}$/i) {
-                sendcontrol "-ERR '$full': command not understood.\r\n";
+                sendcontrol "-ERR Unrecognized command\r\n";
                 last;
             }
 
@@ -3086,7 +3086,7 @@ while(1) {
             $FTPARG="";
         }
         else {
-            sendcontrol "500 '$full': command not understood.\r\n";
+            sendcontrol "500 Unrecognized command\r\n";
             last;
         }