lib/oe/package_manager: Add utils function for multiprocess execution

Our usage of multitprocessing is problematic. In particular, there is a bug
in python 2.7 multiprocessing where signals are not handled until command
completion instead of immediately.

This factors the multiprocess code into a function which is enhanced with
a workaround to ensure immediate signal handling and also better SIGINT
handling which should happen in the parent, not the children to ensure
clean exits. The workaround for the signals is being added to the core
bb.utils function so it can benefit all users.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
package.bbclass: Fixup for using common function

We may as well use the common function for this rather than
duplicating the code.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
debian: fixup

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>