common: Improve waiting for processes on POSIX.

* common/exechelp-posix.c (struct terminated_child): New definition.
(terminated_children): New variable.
(store_result): New function.
(get_result): Likewise.
(gnupg_wait_process): Store results that were not requested and
consider previously stored results.

waitpid(2) may return information about terminated children that we
did not yet request, and there is no portable way to wait for a
specific set of children. As a workaround, we store the results of
children for later use.

• Signed-off-by: Justus Winter <justus@g10code.com>