cryptsetup-generator: add a helper utility to create symlinks

It seems that there's a common pattern among the various generators. Let's add
a helper function for it and make use of it in cryptsetup-generator.

This fixes a bunch of theoretical memleaks in error paths, since *to wasn't
generally freed properly. Not thath it matters.

(cherry picked from commit b559616f)
(cherry picked from commit ea8cb69e)