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 (r265214, r294333, r294407, r294467): misc prop fixes
MFH (r285975, r287143): register mergeinfo for security fixes
MFH (r294497, r294498, r295139): internal documentation
MFH (r294328): upgrade to openssh 6.7p1, re-add libwrap
MFH (r294332): upgrade to openssh 6.8p1
MFH (r294367): update pam_ssh for api changes
MFH (r294909): switch usedns back on
MFH (r294336): upgrade to openssh 6.9p1
MFH (r294495): re-enable dsa keys
MFH (r294464): upgrade to openssh 7.0p1
MFH (r294496): upgrade to openssh 7.1p2

Approved by: re (gjb)
Relnotes: yes

Upgrade to OpenSSH 6.7p1, retaining libwrap support (which has been removed
upstream) and a number of security fixes which we had already backported.

MFC after: 1 week

Vendor import of OpenSSH 6.7p1.