ssh-keyscan: Strictly enforce the maximum allowed SSH2 banner size

From OpenSSH-portable commit ff89b1bed807, OpenBSD commit 6ae664f9f4db.

MFC after: 3 days

(cherry picked from commit 5e5ebbee81bfd1c034caffa00d58d4e06e1b26ee)
(cherry picked from commit 1057339079a0cb37648fa2afe44e9eceec737439)