Tools/scripts/chkversion.pl: Fix and modernize
chkversion.pl was broken in a number of ways, including looking for CVS-style Id
lines in svn headers, not being updated for pkgng, etc.
This commit fixes those, and adopts some modern Perl practices.
NOTE: The $PKG_VERSION env var is renamed to $PKG. This should be a no-op for most
people (in the sense of it didn't work before, and the default should work for
pretty much everybody).