bitbake-worker: Ensure children have default sigterm handler

The children of the worker should have the default SIGTERM handler,
else they'll try and do cleanup which should only happen in the
parent leading to all kinds of bizarre build failures.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
runqueue: Improve sigchld handler

The sigchld handler was reaping any processes and this was leading to
confusion with any other process handling code that could be active.
This patch:

a) Ensures we only read any process results for the worker processes
   we want to monitor
b) Ensures we pass the event to any other sigchld handler if
   it isn't an event we're interested in so the functions are properly
   chained.

Together this should resolve some of the reports of unknown processes
people have been reporting.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
world-image: add extra target

* similar to world target, but collects all PACKAGES
* recipes marked with EXCLUDE_FROM_WORLD are excluded from world as well
  as world-image
* recipes marked with EXCLUDE_FROM_WORLD_IMAGE are excluded completely
  from world-image, sometimes you can build 2 recipes in same sysroot,
  but cannot install them in the same image
* PACKAGES listed in EXCLUDE_PACKAGES_FROM_WORLD_IMAGE are also
  excluded. You can remove e.g. ${PN}-dbg, but keep all other PACKAGES.

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>