[WireGuard] stricter inputs for wg genkey, genpsk, and pubkey

[Daniel Kahn Gillmor]

i've just done a bit of cleanup for wg that makes it stricter for
genkey, genpsk, and pubkey, with the goal of making it explicitly reject
accidental misuse and catch user error.

I've published those changes on the "stricter-inputs" branch on
https://anonscm.debian.org/git/collab-maint/wireguard.git because i'm
not sure whether a series of 6 small changes, with each changeset pretty
small is appropriate for the mailing list.

Please let me know if it's ok to just send this sort of series directly
to the list -- i'd be happy with that approach, but i don't know whether
others on the list would dislike it.

If there was a test suite for the non-kernel parts of wg, i'd normally
integrate failing and succeeding tests as a part of the series, to avoid
a future regression.  Is there interest in a simple test suite for those
parts?

        --dkg