kiss: added KISS_DL to set download tool.

The command is split into words and executed as follows:

$KISS_DL <dest_file> <url>

Examples:

KISS_DL='curl -fLo' (default)
KISS_DL='wget -O'
KISS_DL='aria2c -o'
...

kiss

@@ -302,7 +302,9 @@ pkg_source() {
             log "$1" "Downloading $src"
             mkdir -p "$PWD/$dest"
 
-            curl "$src" -fLo "$_des" || {
+            # Intentional behavior, globbing disabled.
+            # shellcheck disable=2046,2086
+            ${KISS_DL:-curl -fLo} "$_des" "$src" || {
                 rm -f "$_des"
                 die "$1" "Failed to download $src"
             }