Remove duplicate items from $SHELLFU_PATH

src/shellfu.sh.skel

             local incdir
             echo "$SHELLFU_INCLUDE-sh:$SHELLFU_INCLUDE-$SHELLFU_COMPAT:$SHELLFU_PATH" \
               | tr ":" '\n' \
+              | awk '!seen[$0]++' \
               | while read -r incdir;
                 do
                     shellfu __debug "checking: $incdir"