shelltest.od: add more "set -e" tests and add a 'skip' return code.

Based on the earlier t/000-set-minus-e bug in minimal/do on some
shells, let's add some extra tests that reveal the weirdness on those
shells.  Unfortunately because they are so popular (including bash and
zsh), we can't reject them outright for failing this one.

While we're here, add a new return code, "skip", which notes that a
test has failed but is not important enough to be considered a warning
or failure.  Previously we just had these commented out, which is not
quite obvious enough.

...and I updated a few comments while reviewing some of the older
tests.

t/000-set-minus-e/all.do

@@ -1,3 +1,4 @@
+redo-ifchange ../../redo/sh
 rm -f log
 redo fatal >/dev/null 2>&1 || true