gh-156230: Bound a curses window read by the window, not by 2047 (GH-156282)

instr(), in_wstr() and in_wchstr() clamped the count to 2047 and silently
truncated a longer line, which a pad can have.  A window read cannot return
more than the columns left on the line, in the unit each method counts: cells,
characters, or bytes at CCHARW_MAX characters of MB_CUR_MAX bytes per cell.
Cap the count by that, and read the rest of the line when the count is
omitted, which it now can be.

getstr() and get_wstr() read the keyboard rather than the window, so their
limit stays.