Don't use _x_arguments, as we don't support X arguments.
Get rid of -s, because we don't support multiple single-letter options
in one argument.
Add -S, because we ignore options after "--".
Completion now uses "--opt=value" instead of "--opt value". Once the
user presses space and starts a new argument, the option just
completed is out of the picture, whether or not it was given an
argument. This handles options with no arguments or optional arguments
much better; previously, completing such an option would effectively
disable completion for the next argument.
Custom completed options such as "--ao" and friends will no longer
claim to consume an extra argument.
