Making sure that we correctly collect the exit codes from all children
is a bit a hassle. So add another magic igt codeblock for easy forking
and joining.

Note that children are (currently at least) not allowed to call igt_skip.
Add an assert to enforce this.

v2:
- Properly propagate the exit code.
- Fix the segfault.
- Add a child int and num_children paramter to the magic codeblock as
  suggested by Chris Wilson.
- Don't dump noise into stdout when a child thread fails, the parent
  will do that for us already.

v3: Now with some docs.

v4: Fixup igt_waitchildren to properly reset state so it can be used
again.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>