Also refactor New and Get to return pointers to match BaseConf methods.
This makes calling the methods immediately on the return values of the
constructors easier.
Also move config.ConfigFile to buildconfig.go.in. ConfigFile is still
used by wwctl as a default config file to reference; but it is removed
from other locations now.
Removed Persist, as nothing called it.
Signed-off-by: Jonathon Anderson <janderson@ciq.co>
