feat(arp,vipw): fallback to _parse_usage if _parse_help yields nothing

For example on FreeBSD.

Author: scop

completions/arp

@@ -27,7 +27,9 @@ _arp()
     esac
 
     if [[ $cur == -* ]]; then
-        COMPREPLY=($(compgen -W '$(_parse_help "$1")' -- "$cur"))
+        COMPREPLY=($(
+            compgen -W '$(_parse_help "$1" || _parse_usage "$1")' -- "$cur"
+        ))
         return
     fi
 

completions/vipw

@@ -15,7 +15,9 @@ _vipw()
             ;;
     esac
 
-    COMPREPLY=($(compgen -W '$(_parse_help "$1")' -- "$cur"))
+    COMPREPLY=($(
+        compgen -W '$(_parse_help "$1" || _parse_usage "$1")' -- "$cur"
+    ))
 } &&
     complete -F _vipw vipw vigr