Respect old SATURNIN_CONF_PATH when creating new one

This allows prepending config locations to Saturnin.  This should be
normally used only in cases of staged installattions, such as in sanity
tests.

src/saturnin.sh.skel

@@ -408,16 +408,22 @@ saturnin__help() {
 
 saturnin__conf_mkpath() {
     #
-    # Assemble SATURNIN_CONF_PATH from locations $@
+    # Print new $SATURNIN_CONF_PATH by appending locations $@
     #
-    # For each location, print colon-delimited list of directories.  If
-    # location ends with "/ini.d", list of subfolders, sorted by C locale is
-    # printed--this allows for modular configuration.  Otherwise the
-    # location is printed.  Non-existent or non-directory locations are
-    # silently ignored.
+    # Usage:
+    #
+    #     saturnin__conf_mkpath DIR [DIR..]
+    #
+    # First repeat elements of $SATURNIN_CONF_PATH unless it's empty.
+    # Then go through each DIR and print it, unless it ends with "/ini.d",
+    # in which case list its subdirectories, sorted by C locale (this allows
+    # for modular configuration).
+    #
+    # Non-existent or non-directory items are silently ignored.
     #
     local location      # one location argument
     local path          # one path listed
+    test -n "$SATURNIN_CONF_PATH" && echo -n "$SATURNIN_CONF_PATH:"
     for location in "$@";
     do
         test -d "$location" || continue