New config code

[Matt Mackall]

I've just pushed out the long-awaited new config code, along with a fair
amount of ui config clean-up.

Things of note:
 - the undocumented and problematic interpolation syntax is gone
 - the undocumented and problematic : assignment syntax is gone
 - config remembers the ordering of entries
 - config remembers file and line numbers
 - config handles sectionless files so we can use it for templates
 - ui doesn't have a bunch of default args anymore

I've got two experimental patches locally I want some feedback on:

%include file:
 recursively include the given config file with the same trust as the
current file

%unset name:
 unset the given name in the current section, useful for disabling
things in previously read or included config files

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