Fix order of -- and - in cat command

Somewhat unintuitively, the "-" is considered to be a file operand
and not an option, even though it treated to signify standard input
rather than a file named "-". On some systems including macOS,
options to cat cannot follow paths. On those systems, writing "-"
before "--" causes "--" to be treated as a filename, causing the
script to break on macOS.

This changes the order, which lets the cat command work on all
systems, though (as before) if the subsequent argument were a
literal "-" then that would not have the desired effect. In this
case that is okay, since we actually know that the other argument,
the value of the source_file variable, is not exactly "-".

(We even know that it does not begin "-". But keeping the "--"
before it makes clearer to human readers that this argument is a
path and never an option.)

etc/copy-packetline.sh

@@ -101,7 +101,7 @@ function copy_with_header () {
   fi
 
   make_header "$source_file" "$endline" |
-    cat - -- "$source_file" >"$target_file"
+    cat -- - "$source_file" >"$target_file"
 }
 
 function generate_one () {