correct a bug when escaping regex characters

* "\\&" was just turning characters into literal &s, "\\\&" seems to
  work, at least on this OS/awk version.
* Added ) and ] to escaped characters.

z.sh

@@ -142,7 +142,7 @@ _z() {
     if( short == "/" ) return
 
     # escape regex chars in right hand side
-    gsub(/[\(\[\|]/, "\\&", short)
+    gsub(/[\(\)\[\]\|]/, "\\\&", short)
 
     # shortest match must be common to each match
     for( i in matches ) if( matches[i] && i !~ short ) return