When searching for text in file descriptions, always search ext desc

Since users can (and usually do) display extended descriptions in their normal file listings anyway, it makes sense to go ahead and search them for the text (e.g. with the 'F' command from the default command shell File menu/prompt). So the "search/display extended info" prompt has been changed to just "Display extended file info" since it no longer controls whether or not the extended description is searched for the text (it always is). The FL_EXFIND was renamed to FL_EXT and only controls whether the extended info is display (the FL_FIND mode flag, always renamed, must also be specified for a find with extended info).

This closes feature request #285 by Phil (@plt).