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
future-proof.

• All of them include ssh_namespace.h, and include out of

crypto/openssh.

Moving these into crypto/openssh reduces the boilerplate and gives us
a central place if we run into other options like HAVE_LDNS.