MAKEALL: fix kill_children for BSD hosts

ps on BSD hosts (like OS X) do not provide the --no-headers switch nor
understand the AIX format descriptions. Unfortunately there seems no solution to
get the PIDs of children in a platfrom independent manner.
Therefore detect the OS and decide upon that which way to go.

This patch makes the MAKEALL script cleanly stoppable on bare OS X when using
the parallel builds of targets.

Additionally this patch removes double call to grep by a single call to sed for
GNU style child PID detection.

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>