global-functions: add $WaitForFile, wait for file on fetch

The fetch command is asynchronous, the file is not guaranteed to be
available when command terminates.

I opened an issue at Mikrotik support (Ticket#2019041722004999),
their answer:

> You should perform a check in a loop.
> :delay until file exist
>
> That can happen also with any configuration not just files.

So add a function to wait for a file with given name.

I have not seen this with other configuration, though.

daily-psk.template

@@ -10,9 +10,10 @@
 
 :global Identity;
 :global DailyPskMatchComment;
-:global UrlEncode;
 
 :global SendNotification;
+:global UrlEncode;
+:global WaitForFile;
 
 :local Seen [ :toarray "" ];
 
@@ -82,6 +83,7 @@
         :do {
           / tool fetch check-certificate=yes-without-crl \
               $Url dst-path=$Attach;
+          $WaitForFile $Attach;
         } on-error={
           :set Attach "";
         }