Allow the user to override SYMLINK_DIRS with -s

This adds a `-s` that can be used to override the `SYMLINK_DIRS` config,
or the `-S` flag, to lsrc(1), mkrc(1), rcup(1), and rcdn(1).

The `-s` flag is the opposite of -S: any argument, if it is a directory,
is not symlinked but instead recurred down.