Add definition of PATH_MAX on GNU/Hurd

This change remaps the & key. However, / is more relevant for filter and & makes more sense wrt. address.

With the features the readline library provides it would be too naive to ignore.
We break out of the curses mode into the prompt and get back in.
This change introduces dependency on libncurses.

macOS 10.12.4 and many earlier versions have libncurses 5.4 with wide character
support, but not libncursesw.

Update README on dependencies

Add .Ed to match with .Bd

Convert warnings to errors on CI, or they'll go unnoticed.