add --untrusted to showconfig

save settings from untrusted config files in a separate configparser This untrusted configparser is a superset of the trusted configparser, so that interpolation still works. Also add an "untrusted" argument to ui.config* to allow querying ui.ucdata. With --debug, we print a warning when we read an untrusted config file, and when we try to access a trusted setting that has one value in the trusted configparser and another in the untrusted configparser.