Cherry pick updated OpenSSH from the 'msys' branch

This commit essentially updates the OpenSSH files we have in our 'master'
branch from the commit 38e607649(Update OpenSSH to version 6.6b1) on the
'msys' branch.

[jes: cherry-picked from 'msys' and trimmed down to update only the
essential OpenSSH files.]

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge remote-tracking branch 'origin/ssh-6.6p1'

This updates OpenSSH to version 6.6p1, tested by pushing this very
merge...

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>