build: Enable NTVFS file server to be omitted

build: Enable NTVFS file server to be omitted

We now only build it by default with --enable-sefltest, or otherwise
if requested.

The NTVFS file server still has features not present in the smbd file
server, such as a CIFS/SMB proxy, and a radically different design,
but it is also not undergoing any ongoing development so this keeps it
in a safe state for care and maintaince, with less of a security risk
if such an issue were to come up.

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>