New `dm_add_rk()` (referencing key?)

[krlmlr]

Like dm_add_fk() but with arguments reversed -- referenced table first, then referenc_ing_ column, then referencing table. No fourth argument.

Rationale: This allows dm_add_rk() to be copy-pasted from a dm_add_pk() call that is likely to also exist close by in the code.

[jranke]

Good idea, this will make the creation of dm objects easier.