minimal/do: invert the sense of the -c option.

After playing with "continuable" mode for a while, it seems to me that
what most people want will be incremental builds by default (albeit
without any dependency checking to rebuild a target if sources change).
This lets you run minimal/do several times in a row to incrementally
build up your project while you're playing around.  If you want to
start over, use -c.

As a result, in test.do we now recommend using 'minimal/do -c test'
since minimal/do now strays even further from proper redo semantics
otherwise.

While we're here, update the usage message to describe what all the new
options mean.

test.do

@@ -1,3 +1,3 @@
 redo-ifchange _all
 redo t/all
-[ -n "$DO_BUILT" ] || echo "Don't forget to test 'minimal/do test'" >&2
+[ -n "$DO_BUILT" ] || echo "Don't forget to test 'minimal/do -c test'" >&2