win32: shim wopen, and make both open/wopen use _s "secure" variant

win32: shim wopen, and make both open/wopen use _s "secure" variant

The new `__la_wopen` wrapper is a copy of `__la_open` that
expects--rather than converts--a wcs parameter.

The `sopen` variants are offered as "more secure" variants of `open` and
`wopen`; I cannot vouch for their security, but some build systems are
strict about the use of "banned insecure APIs".

I've confirmed that `_wsopen_s` and `_open_s` are present in the Windows
Vista SDK.

I did not confirm that they are available in the Windows XP Platform
SDK, in part because in e61afbd463d1 (2016!) Tim says:

> I'd like to completely remove support for WinXP and earlier.