Attempt to fix mkrc(1) -s

mkrc(1)'s `-s` flag depends on rcup(1) accepting the `-s` flag. Pass it
through, similarly to `-S`.

However, rcup(1) does not yet accept `-s`, so this breaks everything.
rcup(1) takes -s
Passing mkrc(1) tests

Default `force_symlink` to a sentinal that we don't check for.

Fix the test: it moves the directory, then symlinks the files.