Make saturnin subcommand more portable

src/bin/saturnin.skel

 FFOO_PRETTY_USAGE="subcommand"
 SATURNIN_CACHE_HOME="__SATURNIN_CACHE_HOME__"
 SATURNIN_LIBEXEC="__SATURNIN_LIBEXEC__"
+SATURNIN_LIBEXEC_PREFIX="saturnin-"
 SATURNIN_VERSION="__VERSION__"
 
 #

src/ffoo/saturnin_meta.sh

         -mindepth 1 \
         -maxdepth 1 \
         -executable \
-      | sed -e 's/.*saturnin-//' \
+      | sed -e "s|^.*/||; s|^$SATURNIN_LIBEXEC_PREFIX||" \
       | sort
 }
 
 }
 
 saturnin_runsc() {
-    local lexpath="$SATURNIN_LIBEXEC/saturnin-$SATURNIN_SUBCOMMAND"
+    local lexpath
+    lexpath+="$SATURNIN_LIBEXEC/"
+    lexpath+="$SATURNIN_LIBEXEC_PREFIX$SATURNIN_SUBCOMMAND"
     debug -v lexpath
     debug "\$*='$*'"
     test -x "$lexpath" || {