minimal/do: fix a really scary bugs in "set -e" behaviour.

If you run something like

  blah_function || return 1

then everything even *inside* blah_function is *not* subject to the "set -e"
that would otherwise be in effect.  That's true even for ". subfile" inside
blah_function - which is exactly how minimal/do runs .do files.

Instead, rewrite it as

  blah_function
  [ "$?" = "0" ] || return 1

And add a bit to the unit tests to ensure that "set -e" behaviour is enabled
in .do files as we expect, and crash loudly otherwise.

(This weird behaviour may only happen in some shells and not others.)

Also, we had a "helpful" alias of redo() defined at the bottom of the file.
Combined with the way we use '.' to source the .do files, this would make it
not start a new shell just to run a recursive 'redo' command.  It almost
works, but this stupid "set -e" bug could cause a nested .do file to not
honour "set -e" if someone ran "redo foo || exit 1" from inside a .do
script.  The performance optimization is clearly not worth it here, so
rename it to _redo(); that causes it to actually re-exec the redo program
(which is a symlink to minimal/do).

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

@@ -0,0 +1 @@
+rm -f log *~ .*~