This adds support for detecting incorrect version numbers and links
in the ports list.
Also, unlike before it doesn't parse the package.sh script but executes
it instead which allows us to detect syntax errors.
The new message for skipping builds makes it hard to
distinguish failed builds from builds that were just skipped
so change it back to the old one - but don't actually build
packages twice again.
Previously we'd end up building some ports multiple times, e.g.
as a dependency for another port. This changes the build_all.sh
script so that it builds ports only once.
This patch allows for a verbose argument to be passed
so that the build output of the individual builds
is printed to stdout instead of /dev/null to help with diagnosing errors
If the verbose argument is not passed the old behaviour is preserved
and the build output is printed to /dev/null
