the common/Make.rules file into common/Version so that libapparmor's
configure.in can make use of it, meaning there's one less thing to
adjust when updating the version. It also bumps the trunk version
from 2.5 to 2.5.90 in (perhaps excessively long) preparation for the
2.6.0 release, and to indicate that it's newer than the 2.5.x branch.