tests/test-gendoc-el.t

rust-config: add support for default config items

Now that configitems.toml exists, we can read from it the default values for
all core config items.

We will add the devel-warning for use of undeclared config items in a later
patch when we're done adding the missing entries for `rhg`.

#require docutils gettext

  $ $TESTDIR/check-gendoc el
  checking for parse errors