driver: Stop requiring function_argzero.

Instead, use the %N prompt expando, as suggested by Bart in users/21637.

Avoid the `print -P %N` syntax to save a fork on Cygwin, at the expense
of not supporting zsh's from 2001 through c. 2004 vintages.

Fixes #338.

zsh-syntax-highlighting.plugin.zsh

@@ -1 +1,2 @@
+0=${(%):-%N}
 source ${0:A:h}/zsh-syntax-highlighting.zsh