[LTP] waitpid: new API

[Stanislav Kholmanskikh]

Hi!

This series is a continuation of [1], i.e. I converted those waitpid*
test cases utilizing wait_for_parent() to use the checkpoint interface
(along with the new test API).

The main difference from [1] is that I moved duplicated code to
a common header.

The only unconverted test case is waitpid10. It needs some more time.

Thanks.

[1] http://lists.linux.it/pipermail/ltp/2016-July/002178.html