Add a cool new system callback function to PBIs on 10.x and beyond

This allows PBIs, which run in a container, to use the pbisyscmd, openwith
or xdg-open, and "talk" to applications on the outside of the container.

The use-case for this would be a application like Thunderbird, which may
want to use the "xdg-open" command to open a PDF file with something else
on the system.

It could be used by other apps, such as terminal emulators to run a shell on the system,
even though the GUI is run in the container
Merge branch 'master' of github.com:pcbsd/pcbsd