zsh: support site specific files

This fixes the `site-scripts` and `site-functions` directories
so that zsh points to the HOMEBREW_PREFIX version of these files
instead of the Cellar version.

This means you can drop files in these directories and the files
will persist when you upgrade zsh. Otherwise they disappear when
you upgrade zsh.

Closes Homebrew/homebrew#11955.

[jn: mkpath everything under share/zsh]

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/keg.rb

@@ -80,6 +80,7 @@ class Keg < Pathname
       when INFOFILE_RX then ENV['HOMEBREW_KEEP_INFO'] ? :info : :skip_file
       when LOCALEDIR_RX then :mkpath
       when *share_mkpaths then :mkpath
+      when /^zsh/ then :mkpath
       else :link
       end
     end