Fix up libsecret licensing

 * License libsecret under LGPL2.1+. We can do this because the
   files that make up the library are either LGPL2.0+ or LGPL2.1+
 * Correctly include a file which contains the full texts of the
   various licenses used in the tests.
 * Include license headers in some files that contained no license
   headers.

https://bugzilla.gnome.org/show_bug.cgi?id=680781