Update DX_COMMAND_OUTPUT to handle newlines and backslashes.

The 'read' builtin eats backslashes from the input, so this macro fails
to properly handle output containing backslashes.  Since the -r option
is not portable according to the Autoconf manual, fix up the input by
escaping all backslashes.

While we're at it, remove the "feature" which converts newlines to
spaces because it seems rather silly.