Explicitly mention that the rpmio/ sub dir is under LGPL

As the code in the rpmio sub directory was split out of the lib sub dir
it is already under LGPL as "code derived from" "the source code in the
lib subdirectory" according to the license. But not having the sub directory
mentioned in the license confuses users and contributers.

The original release tarballs in http://ftp.rpm.org/releases/historical/ show
the license was changed into the existing dual one between rpm 2.4.3 and 2.4.4,
and that no rpmio/ directory exists at that time. Our git repo disagrees with
the time of rpmio/ split due to some conversion artifacts (cvs to mercury to
git), as it shows rpmio/ directory existing from the first commit, but this was
not actually the case.

The license stating that the dual license is there to allow linking with librpm
from non-GPL code supports this interpretation as librpmio is required in order
to use librpm even if it is a separate library nowadays.

So this change does not change the license of any code but only clearifies the
current situation.

Resolves: #516

COPYING

@@ -2,8 +2,8 @@ RPM is covered under two separate licenses.
 
 The entire code base may be distributed under the terms of the GNU General
 Public License (GPL), which appears immediately below.  Alternatively,
-all of the source code in the lib subdirectory of the RPM source code
-distribution as well as any code derived from that code may instead be
+all of the source code in the lib and rpmio subdirectories of the RPM source
+code distribution as well as any code derived from that code may instead be
 distributed under the GNU Library General Public License (LGPL), at the
 choice of the distributor. The complete text of the LGPL appears
 at the bottom of this file.