How does the following addition to bsd.pkg.mk look?  There would, of
course, be appropriate changes to mk.conf.example and packages(7).

The pkg Makefile would define USE_SSH and the build system would put SSH
in the environment and pull in needed depends.  Defaults would be provided
but the user could specifiy PKG_SSH= one of {openssh,ssh,ssh6, or the full
path to someother ssh program}.

.if defined(USE_SSH)
# first provide defaults.  If we have ssh in the base system
# use that as a default, otherwise use the security/ssh package
# as a default.
.if exists(/usr/bin/ssh)
PKG_SSH?=	/usr/bin/ssh
.else
PKG_SSH?=	ssh
.endif
# now pull in any depends we may need and provide the full path
# to ssh.
SSH=	${LOCALBASE}/bin/ssh
.if  (${PKG_SSH} == "openssh")
DEPENDS+=       openssh-*:../../security/openssh
.elif  (${PKG_SSH} == "ssh")
DEPENDS+=       ssh-*:../../security/ssh
.elif  (${PKG_SSH} == "ssh6")
DEPENDS+=       ssh6-*:../../security/ssh6
.else
SSH=	${PKG_SSH}
.endif
MAKE_ENV+=      SSH="${SSH}"
.endif



-dan