Experimental -S for mkrc

Unlike the `-S` for `lsrc`, `rcup`, `rcdn`, this `-S` sets a Boolean:
either you do want these files symlinked, or you don't.

Unsure whether it works, because it depends on `rcup` supporting this.
Add a -s to mkrc

Allow the user to later override their `-S` decision with `-s`. The
default is `-s`.