Renamed core library "basic.sh" to "core.sh"
Do not catch error when Bash "source" fails

This gives a much better stack trace.
Cosmetic changes for stacktrace output
Extended documentation and usage message
Renamed options "PKGBOX_OPTS" to "O" and changed build directory layout
Fixed test broken by commit 4bb431f
Updated submodule pkg (datag/pkg-packages)