The bug was caused by buffered output. Flushing resolved it.
We still recommend to allways call plan().
  
Also fix some compile warnings (formal parameter different from declaration)


unittest/examples/Makefile.am:
  Omit core-t, since it will always fail.
unittest/examples/no_plan-t.c:
  Comment that we recommend calling plan(NO_PLAN)
unittest/mytap/tap.c:
  Use the named constant NO_PLAN
  Flush all output.
unittest/mytap/tap.h:
  Change documentation for the plan() function.