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.