It's important to make sure that we don't change a config_mgr_t
after we start using it to make objects, or we could get into
inconsistent states. This feature is the start of a safety
mechanism to prevent this problem.
Remember that our goal in the present refactoring is to allow each
subsystem to declare its own configuration structure and
variables. To do this, each module will get its own
config_format_t, and so we'll want a different structure that wraps
several config_format_t objects. This is a "config_mgr_t".
Now that we have a reasonable implementation for overriding the
default options for TestingTorNetwork, we don't need to modify
config_var_t structs any more. And therefore, we can have constant
format options, like reasonable people.
Right now, this has been done at a high level by confparse.c, but it
makes more sense to lower it.
This API is radically un-typesafe as it stands; we'll be wrapping it
in a safer API as we do #30914 and lower the struct manipulation
code as well.
Closes ticket 30864.
