Make package listing globally available to checks.

This adds two new variables:

$PKG_LISTING, which contains a simple file listing
$PKG_DETAILED_LISTING, which contains the file list with permissions
and ownership information
Add command line option to exclude checks from the reporting.
Fix errors and warnings not being counted properly.

Adjust the for-loops to while-loops to fix handling of strings
with spaces caused them to be executed in a subshell, thus all
modifications to $ERRORS and $WARNINGS were reverted when we
left the loop.

Deep dive into bash syntax semantics to fix that...