Allow newlines to be in values, but stop searching for assignments
once an unescaped/unquoted newline is discovered. This is preparation
to using `parse_assignments` to process the `--options` command-line
argument, which might have embedded newlines.
* Ignore empty `key=value` strings in variable assignments. Now the
following lines are supported with the expected meaning:
* `a=1 , b=2`
* `a=1,,b=2`
* Improve the warning message when there is an unterminated quote.
* Add a warning if the line ends with a backslash.
Also add unit tests.
