menuconfig: Add show-name mode + small help dialog improvement

 - Pressing 'c' ('n' was taken) toggles a mode where the names of all
   symbols are shown before their prompts. This saves going into the
   help display when you just want the name.

 - The name of the symbol/choice now appears in the main text of the
   help dialog rather than in the title, where it might be hard to
   spot, as pointed out by Carles Cufi.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>