ncursesw: override bionic MB_LEN_MAX and MB_CUR_MAX

add custom string mb to wc convert funcs

bionic defines those values to 1, which is not compatible
with our implementation of wchar_t

Signed-off-by: Tanguy Pruvot <tanguy.pruvot@gmail.com>

Change-Id: I44b0a10830946aa2201641759dae5cffb43f646d
update widechar part and tests from ncurses 5.9

Change-Id: I63e1ba135c65e4818b4891763cc6999510a3a0dd