Pretty much all applications/libs built out of openssh define a common
set of options:
- HAVE_LDNS is only relevant for applications that build readconf.c and
libssh, which builds getrrsetbyname off of ldns, but defining it is more
- All of them include ssh_namespace.h, and include out of
Moving these into crypto/openssh reduces the boilerplate and gives us
a central place if we run into other options like HAVE_LDNS.