bash-completion: use a better definition of __contains_word

- scope the iterator var
- use the correct, quoted, non-expansion prone positional parameter
  notation
- prevent expansion on RHS of comparison
- remove unneeded explicit returns.

This really should be defined only once...

shell-completion/bash/systemctl

@@ -32,9 +32,10 @@ __systemd_properties() {
 }
 
 __contains_word () {
-        local word=$1; shift
-        for w in $*; do [[ $w = $word ]] && return 0; done
-        return 1
+        local w word=$1; shift
+        for w in "$@"; do
+                [[ $w = "$word" ]] && return
+        done
 }
 
 __filter_units_by_property () {