Log Message:
show-all: fix output for list variables containing dollar characters

Before, variables containing dollar characters displayed so wrong that it
was hard to explain.

To fix the problem, I typed almost random characters into the code until
the output was exactly as expected. I still do not understand:

* why the list variables need 8 dollars to survive the @x@ loop,
* why the code only works if the dollars come from an external variable
  instead of being written inline,
* why the backslash in the :C modifier needs to be doubled.

Anyway, the output of "bmake show-all-extract" now contains the shell
variable $${extract_file}, just as it should. The dollars are now doubled
in the output and thereby match the source code from the Makefile
exactly.