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

[Jason A. Donenfeld]

Hey Daniel,

You can always post massive patch sets to the list, so long as you do
it with git-send-email so that the threading works correctly.
Sometimes it's nice to be able to comment on patches via inline
replies. The lists.zx2c4.com projects generally operate the same way
as LKML. Alterntively (or additionally) you can use git-request-pull,
which is often easier for actually merging the commits. Anyway, I'll
review stricter-inputs shortly.

Jason