TOOLS/zsh.pl: don't consume extra arguments

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.