Introduce find_exe. Use instead of find_in_path for programs. The behaviour of find_in_path was broken for config options containing path names, because it always searched the given path, even when not necessary. The find_exe function is more polite: if the name passed to it contains a path component, it just returns it.

ui: Rename has_config to has_section.

patchbomb: Defer the import of readline. If imported at the top of the module, the import appears to succeed, but raw_input doesn't acquire magic editing fu. I suspect this has something to do with the newish demandimport code, because the prior code worked with demandload.