run-command: add first helper for pp child states

run-command: add first helper for pp child states

There is a recurring pattern of testing parallel process child states
and file descriptors to determine if a child is running, receiving any
input or if it's ready for cleanup.

Name the pp_child structure and introduce a first helper to make these
checks more readable. Next commits will add more helpers and checks.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>