Upgrade OpenSSH to 7.3p1.

This is the last version of OpenSSH which does not break compatibility
more than we can live with in a stable branch. Further commits will
follow to backport some bug fixes from newer versions.

The sshd breakage in the previous attempt was due to an upstream bug
(a 0 was changed to a 1 while refactoring send_rexec_state() in sshd.c)
which only manifested itself when sshd was built with SSH 1 support.

Approved by: re@

Revert OpenSSH 7.3p1; something went wrong between testing and committing.

Approved by: re@

Upgrade OpenSSH to 7.3p1.

This is the last version of OpenSSH which does not break compatibility
more than we can live with in a stable branch. Further commits will
follow to backport some bug fixes from newer versions.

Approved by: re@

MFH (r263234, r263691, r266465, r290671, r290672, r290673, r290674,
r294320, r294322, r294324, r294330, r294469, r294494, r294466)

Reduce diffs to head in preparation for removing HPN and None.

MFH (r261320): upgrade openssh to 6.5p1
MFH (r261340): enable sandboxing by default

MFH (r261320): upgrade openssh to 6.5p1
MFH (r261340): enable sandboxing by default