showlinenum : Fix lint warning when padding

- Convert argument explicitly to a string before passing to length().

Prior to this change gawk --lint would warn that length() "received
non-string argument".
showlinenum : Flush stdout before FATAL to stderr

- (FATAL) Flush stdout with an empty string before outputting to stderr.

This should also make the error output easier to read since in most
cases the last character sent to stdout before calling FATAL() is
probably a linefeed (ie it's already been flushed) and printing an
empty string after that will appear as a blank line separating the
regular output from the error message.