Attempt to work around macOS vfork bug

Attempt to work around macOS vfork bug

Problem reported by YAMAMOTO Mitsuharu in:
http://lists.gnu.org/archive/html/emacs-devel/2017-05/msg00342.html
This is related to the fix for Bug#26397.
* src/callproc.c (call_process_cleanup, call_process) [!MSDOS]:
Report internal error if wait_for_termination fails.
* src/sysdep.c (get_child_status): Return -1 if waitpid is
buggy, instead of aborting.
(wait_for_termination): Return bool success value.
All callers changed.