Reorder the broctl process command Bro arguments

Moved custom scripts to the end of the Bro command-line to avoid
getting a syntax error with scripts that use commands outside of
an event handler.  Also added a check if the specified trace file
exists and don't bother running Bro if it does not.

Addresses BIT-1124
Improve broctl help message for the process command

Improved description of the "process" command to avoid confusion (it now
shows the "--" argument to separate the bro options from the bro scripts).
Also reformatted the output slightly so it fits in fewer lines and avoid
line wrap on 80-column display.
Update and improve the tests of broctl process
Fix error handling for process command
Merge remote-tracking branch 'origin/topic/dnthayer/ticket1124'

* origin/topic/dnthayer/ticket1124:
  Fix error handling for process command
  Update and improve the tests of broctl process
  Improve broctl help message for the process command
  Reorder the broctl process command Bro arguments

BIT-1124 #merged