This needs to be a point so that the CONF_VAR() macro can work
correctly.

Some of these helpers will be needed in multiple places in the unit
tests, so we should move them now.

This commit moves VersioningAuthoritativeDirectory,
RecommendedClientVersions, and RecommendedServerVersions.

This way, we can't get out of sync between the two declarations.

I've chosen the "AuthDirMaxServersPerAddr" option here for
simplicity, since it is used literally nowhere else besides the dirauth
module.  Once we have all the infrastructure in place for this, we
can move more options into this structure.

When a subsystem is disabled, there will be no corresponding object
allocated, and no magic numbers on it.

Like "obsolete" variables, these variables produce a warning when
you try to set them, but the warning says that the relevant module
doesn't have support.

The confdecl macros now have a CONF_CONTEXT that you can define to
make all the modules in a given table disabled.

These modules are only built when the selected modules are disabled.
The provide stub implementations of the subsystem blocks.  Later,
other stub implementations could move here.

Having real subsystem blocks here will let us handle disabled
configuration options better.