process command misplaces custom scripts

If you run "broctl process" with a custom script, then you need
to specify "--" so that broctl knows where the Bro options end
and where the custom scripts begin. So your example
should be:
broctl process trace.pcap – test.bro

Doing so moves test.bro farther down towards the end
of the Bro command line.

However, in this example we still get a different error,
because broctl adds its own script (process-trace.bro,
which contains a couple of redefs) at the very end of
the Bro command. I will change the order so that the
process-trace.bro is before the custom scripts (which
solves the syntax error).

In branch topic/dnthayer/ticket1124, I've changed the order of
scripts so that user-specified scripts are always at the end of
the Bro command, and I've improved the broctl help message
to show how the process command should be used.