SFI: improved record field name canonicalization code
SFI: improved record field name canonicalization code

- use g_strdup for clarity
- fix names/comments: legal -> valid

swesterfeld/beast,GNOME/beast,GNOME/beast,swesterfeld/beast,GNOME/beast,swesterfeld/beast,tim-janik/beast,tim-janik/beast,GNOME/beast,swesterfeld/beast,tim-janik/beast,tim-janik/beast,tim-janik/beast,GNOME/beast,GNOME/beast,GNOME/beast,tim-janik/beast,swesterfeld/beast,swesterfeld/beast,tim-janik/beast,swesterfeld/beast