Parameterize atf-sh in atf-sh's tests

Build upon f43cc24dd97717159bdc04f8f4a90f483515b984 to parameterize two
more cases of direct calls to atf-sh.
Release atf-0.21

Update release notes accordingly and bump the major version of the shared
libraries.

In theory, bumping the major version is not strictly necessary because we
only removed the internal config module... but there might be binaries out
there relying on it (see the old version of the ATF libraries in NetBSD).
Better be safe than sorry given that a symbol removal requires a major
version bump.