include/conf: Add support for multiple configuration files

include/conf: Add support for multiple configuration files

The current conf implementation does not permit processes to use
more than one configuration file.
This commit modifies the conf module so that each process may use
an arbitrary number of configuration files by appending the name of
a config domain to the commandline of commands such as conf_get()
and conf_set(), falling back to "default" if the config domain was
omitted.
This commit further adds the conf_get_domains() method that may be
used to query the names of known configuration domains.