Allow overriding generator, selector and consumer on command line

Allow overriding each of the essential parts at CLI, in which case do
not load them from INI.

src/libexec/saturnin-menu

 ffoo import pretty
 
 usage() {
-    mkusage "[name]"
+    mkusage "[-g generator_cmd] [-s selector_cmd] [-c consumer_cmd] [name]"
 }
 
 SATURNIN_MENU_MNAME=""
 
 load_parts() {
     #
-    # Load mandatory parts
+    # Load mandatory parts for menu name $1
     #
     local mname
-    mname=$(get_mname) || usage
+
+    # last chance to get mname
+    test -n "$SATURNIN_MENU_MNAME" \
+     || SATURNIN_MENU_MNAME="$(get_mname)" \
+     || usage
+    mname="$SATURNIN_MENU_MNAME"
+
+    # check that menu item exists
     inigrep -s menu.$mname | grep -q . \
      || die "no such menu: $mname"
-    SATURNIN_MENU_GENERATOR="$(get_generator)"  \
+
+    test -n "$SATURNIN_MENU_GENERATOR" \
+     || SATURNIN_MENU_GENERATOR="$(get_generator)"  \
      || die "cannot find generator for menu: $mname"
-    SATURNIN_MENU_SELECTOR="$(get_selector)" \
+
+    test -n "$SATURNIN_MENU_SELECTOR" \
+     || SATURNIN_MENU_SELECTOR="$(get_selector)" \
      || die "cannot find selector for menu: $mname"
-    SATURNIN_MENU_SELECT_ARGS="$(get_select_args)"
-    SATURNIN_MENU_CONSUMER="$(get_consumer)" \
+
+    test -n "$SATURNIN_MENU_SELECT_ARGS" \
+     || SATURNIN_MENU_SELECT_ARGS="$(get_select_args)"
+
+    test -n "$SATURNIN_MENU_CONSUMER" \
+     || SATURNIN_MENU_CONSUMER="$(get_consumer)" \
      || die "cannot find consumer for menu: $mname"
 }
 
 get_mname() {
     #
-    # If not set, take from global INI default
+    # Take from global INI default
     #
-    test -n "$SATURNIN_MENU_MNAME" && {
-        printf '%s' "$SATURNIN_MENU_MNAME"
-        return 0
-    }
     inigrep -1 -p menu._default_.name | grep .
 }
 
     eval "$SATURNIN_MENU_CONSUMER"
 }
 
-SATURNIN_MENU_MNAME="$1"
+while true; do case "$1" in
+    -g) SATURNIN_MENU_GENERATOR="$2"; shift; shift ;;
+    -s) SATURNIN_MENU_SELECTOR="$2"; shift; shift ;;
+    -c) SATURNIN_MENU_CONSUMER="$2"; shift; shift ;;
+    -*) usage ;;
+    "") usage ;;
+    *)  SATURNIN_MENU_MNAME="$1"; shift; break ;;
+esac done
 
 load_parts