Explicitly terminate option list of `printf'

This prevents `printf' from choking on prefixes starting with '-'.
_msg() being a "private" function, this does not really matter, but
when it comes to expression, the more accurate, the better.

ffcast.bash

@@ -39,7 +39,7 @@ _msg() {
     shift || return 0
     local fmt=$1
     shift || return 0
-    printf "$prefix$fmt\n" "$@"
+    printf -- "$prefix$fmt\n" "$@"
 }
 
 debug() {