db-check-package-libraries: Make it more robust.

First, to ease debugging, put some information in tmpdir filenames.

Then, to avoid issues with reading files that are mode unreadable, or
stat()ing them in unexecutable directories, add two `find` commands
that reset bad permissions to 755/644.

Then, skip over symlinks.  Beside absolute links being broken in our
implementation, it also means that links to files in different
packages are broken.  This leads to an exception being thrown, as the
file can't be open()ed.

Finally, skip over non-regular-file files.  One package I tried
contained a named FIFO, which caused it to hang on open().

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