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.

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Unbreak the WITHOUT_KERBEROS build and try to reduce the odds of a
repeat performance by introducing a script that runs configure with and
without Kerberos, diffs the result and generates krb5_config.h, which
contains the preprocessor macros that need to be defined in the Kerberos
case and undefined otherwise.

Approved by: re (marius)