ssh: move common Makefile boilerplate to a new ssh.mk

This moves SSHDIR and ssh_namespace.h handling to a common location,
and will simplify future work such as adding U2F support (D32509).

Reviewed by: kevans
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32808