MINOR: mworker-prog: stop old programs in mworker_ext_launch_all

MINOR: mworker-prog: stop old programs in mworker_ext_launch_all

This patch is a part of series to reintroduce the program support in the new
master-worker architecture.

Now, after refactoring in master-worker mode it's the master process, who
stops workers forked before the reload. Current worker no longer sends USR1 or
TERM signals to the previous one after ports binding. This behaviour is kept
only for the standalone mode.

So, in case of programs, it's up to master process as well to stop programs,
which were launched before reload. Let's do this in mworker_ext_launch_all(),
just before starting the new programs.