Improve correctstatus speed

Use "apt-cache show" instead of "dumpavail" in the Makefile.

Using "dumpavail" dumps all packages, while we're only interested in one
specific package. This wasts a log of cycles and time when many packages
need fixing.

Use "show $pkg" to directly dump only the requested package.

Use "--no-all-versions" to show only the candidate for installation.

Closes: #759343. Thanks to Philipp Hahn for the patch.