Put user's PATHs at end of superenv PATH

Making sure to remove X11, should be safe if this order is kept.

Refs Homebrew/homebrew#14519.
Fixes Homebrew/homebrew#14544.

Library/Homebrew/superenv.rb

@@ -104,6 +104,7 @@ class << ENV
     paths << HOMEBREW_PREFIX/:bin
     paths << "#{MacSystem.x11_prefix}/bin" if x11?
     paths += %w{/usr/bin /bin /usr/sbin /sbin}
+    paths += ORIGINAL_PATHS.map{|pn| pn.realpath.to_s rescue nil } - %w{/usr/X11/bin /opt/X11/bin}
     paths.to_path_s
   end
 
@@ -213,7 +214,7 @@ end
 
 class Array
   def to_path_s
-    map(&:to_s).select{|s| s and File.directory? s }.join(':').chuzzle
+    map(&:to_s).uniq.select{|s| File.directory? s }.join(':').chuzzle
   end
 end