Show only sub-commands starting with SATURNIN_LIBEXEC_PREFIX

This was always intended; sub-commands starting with dedicated prefix
is the mandatory convention.

src/saturnin.sh.skel

         -mindepth 1 \
         -maxdepth 1 \
         -executable \
+        -name "$SATURNIN_LIBEXEC_PREFIX*" \
       | sed -e "s|^.*/||; s|^$SATURNIN_LIBEXEC_PREFIX||" \
       | sort
 }