libexif: fix CVE-2018-20030

Also:

- Use GitHub as source for CVE-2017-7544.patch [0]. The resulting patch
  is identical, but comes in a different format.

- Update the website, as http://libexif.sourceforge.net/ shows only a
  move notice.

- Add erictapen as maintainer.

[0] https://github.com/libexif/libexif/commit/c39acd1692023b26290778a02a9232c873f9d71a