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.
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.
