Merge branch 'la/hide-trailer-info'

Merge branch 'la/hide-trailer-info'

The trailer API has been reshuffled a bit.

* la/hide-trailer-info:
  trailer unit tests: inspect iterator contents
  trailer: document parse_trailers() usage
  trailer: retire trailer_info_get() from API
  trailer: make trailer_info struct private
  trailer: make parse_trailers() return trailer_info pointer
  interpret-trailers: access trailer_info with new helpers
  sequencer: use the trailer iterator
  trailer: teach iterator about non-trailer lines
  trailer: add unit tests for trailer iterator
  Makefile: sort UNIT_TEST_PROGRAMS