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.

Fixes zsh-users/zsh-syntax-highlighting#292 (compatibility with zsh-5.1.1/msys2 on windows).