save userconfig as text instead of a cfg_obj tree

save userconfig as text instead of a cfg_obj tree

once the user configuration has been merged into the effective
configuration, it no longer needs to be accessed as a configuration
tree, but we still want to be able to show it with 'rndc showconf -user'.

because the recursive strucure of cfg_obj objects is fairly large, the
canonical text form is a fraction of the size of the configuration
tree, so we now save it in that form instead.