Merge branch 'la/trailer-cleanups'

Code clean-up.

Keep only the first three clean-ups, and discard the rest to be replaced later.
cf. <owly1qetjqo1.fsf@fine.c.googlers.com>
cf. <owlyzg1dsswr.fsf@fine.c.googlers.com>

* la/trailer-cleanups:
  trailer: split process_command_line_args into separate functions
  trailer: split process_input_file into separate pieces
  trailer: separate public from internal portion of trailer_iterator