openssh: Support building with MIT KRB5

[cy]

Summary

Remove HEIMDAL=1 from openssh/krb5_config.h and move the definition
to the Makefile in order to control whether we're building under
Heimdal or MIT.

Add MIT KRB5 LIBS and INCLUDES to the openssh build.

Sponsored by: The FreeBSD Foundation

Test Plan

Running here for about a month now.

[cy]

-U99999 version.

[markj]

secure/ssh.mk

14

I believe we want to always compare with "no", so this should be ${MK_MITKRB5} == "no".

[cy]

Applied markj@'s comment to compare MK_MITKRB5 against "no".