TODO: check for non-executable binaries in /bin /sbin /usr/bin /usr/sbin.
probably also non-executable shared libs.

Future options:

-e  Only log errors, not warnings (right now, everything is a warning).

Future test ideas:

- more forbidden files. for fonts.{dir|scale}, we need the doinst.sh
  test (doinst can and should create these; they should *not* just be
  files in the package).

- static libraries? some packages ship these because upstream doesn't
  support shared libs, though. maybe only complain if libfoo.so.* and
  libfoo.a both exist (if we have a shared lib, we shouldn't also have
  a static one). Maybe this should be a disable-able warning?

- duplicate files, maybe the error message could suggest a "ln -s" command
  to use if the file really does need to appear in multiple dirs.

- fonts. make sure they are what their filename says, and are installed to
  the correct /usr/share/fonts/* or /usr/share/kbd/consolefonts dir.

Other ideas:

When linting multiple packages, print a summary: 100 packages checked,
10 failed, 90 passed. Maybe with percentages too.

Clean up the output. Make it easier to grep.