Merge branch 'ab/usage-die-message'

Merge branch 'ab/usage-die-message'

Code clean-up to hide vreportf() from public API.

* ab/usage-die-message:
  config API: use get_error_routine(), not vreportf()
  usage.c + gc: add and use a die_message_errno()
  gc: return from cmd_gc(), don't call exit()
  usage.c API users: use die_message() for error() + exit 128
  usage.c API users: use die_message() for "fatal :" + exit 128
  usage.c: add a die_message() routine