ui: add new config flag for interface selection
This patch introduces a new config flag ui.interface to select the interface
for interactive commands. It currently only applies to chunks selection.
The config can be overridden on a per feature basis with the flag
ui.interface.<feature>.
features for the moment can only be 'chunkselector', moving forward we expect
to have 'histedit' and other commands there.
If an incorrect value is given to ui.interface we print a warning and use the
default interface: text. If HGPLAIN is specified we also use the default
interface: text.
Note that we fail quickly if a feature does not handle all the interfaces
that we permit in ui.interface; in future, we could design a fallback path
(e.g. blackpearl to curses, curses to text), but let's leave that until we
need it.