The "^_" has always been poorly legible (the underscore melting into
the bottom of the terminal) and difficult to type (Shift+Ctrl+minus).
The "^/" is both more readable and easier to type.
In terminal emulators, "^/" is shown, but on a Linux console "^-",
as ^/ does a backspace in the default console key mapping.
Also, allow using "^-" when rebinding ^_ in a nanorc file.
This addresses https://savannah.gnu.org/bugs/?57393.
d1957819c4