gh-154855: Ask non-ncurses curses for one more character (GH-154870)

Passing n to the library is ncurses' reading of n: it stores n characters
and adds a terminator.  NetBSD curses counts the terminator in n.

Ask a library that is neither ncurses nor PDCurses for n + 1, and read
again if it stored more than asked; truncating could split a multibyte
character.  This is not possible for input, so getstr() and get_wstr() are
left as they are.

instr() now takes the length from the value returned by winnstr(), as
X/Open specifies, instead of searching for a terminator which it does not.