Avoid inconsistent macro option string handling when headless

[imagejan]

People keep stumbling over inconsistencies in option string parsing between ImageJ 1.x and the headless mode. While both "key=variable" and "key=&variable" seem to work in the ImageJ GUI, headless mode requires proper string concatenation: "key=" + variable + "".

See also this forum thread.

If things cannot be kept consistent, there should at least be a more informative message for users.