headers/stralign{,_s}: Fix implementations of `ua_wcs{cpy_s,chr,rchr}`

Old code checked for `_WConst_Return` which is never defined; it is
`_WConst_return` that is defined in corecrt.h, so these definitions were
never compiled.

And these definitions turn out to be incorrect: `ua_wcscpy_s` references
non-existent parameters, and `ua_wcschr` calls `wcscpy`. These have to be
re-implemented.

It's worth noting that Microsoft stralign.h has also been having these typos
since more than twenty years ago.

Signed-off-by: LIU Hao <lh_mouse@126.com>