Allow importing of fewer than 3 secret keys for partial signing

[a5an0]

A L3 user might want to have their seed packets spread out far apart, and not want to have to get three of them together in the same place at the same time. It would be great to create a psbt on the primary laptop, and then travel to one seed packet with the secondary laptop, sign the psbt, travel to the second location, sign it, and so forth.