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.

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