Use SecureZeroMemory in memwipe on Windows

[teor]

In #7419, we use explicit_bzero/memset_s in memwipe when available.

Windows has SecureZeroMemory with the same semantics in Windows.h.
​​https://msdn.microsoft.com/en-us/library/windows/desktop/aa366877.aspx

As long as we build our Windows binaries with headers that contain SecureZeroMemory, the code will work on any Windows version:

"This function is defined as the RtlSecureZeroMemory function (see WinBase.h). The implementation of RtlSecureZeroMemory is provided inline and can be used on any version of Windows (see WinNT.h.)"

[rl1987]

​https://github.com/rl1987/tor/compare/securezeromemory

[teor]

Looks great, I don't have Windows so can't compile it, but it seems simple enough, let's get it tested, then merged.

This is going to require a manual merge with #7419 that preserves all the different cases.

(Edit: explain *why* I didn't compile it)

[nickm]

Merged into master after #7419, with a tiny amount of cleanup.

[teor]

Jenkins says:

../tor/src/common/crypto.c: In function 'memwipe':
22:31:43 ../tor/src/common/crypto.c:2965:3: error: implicit declaration of function 'SecureZeroMemory' [-Werror=implicit-function-declaration]
22:31:43 ../tor/src/common/crypto.c:2965:3: error: nested extern declaration of 'SecureZeroMemory' [-Werror=nested-externs]

​https://jenkins.torproject.org/job/tor-ci-windows/1795/consoleFull

Microsoft says we need to include Windows.h (which includes WinBase.h) to get this function.
​https://msdn.microsoft.com/en-us/library/windows/desktop/aa366877.aspx

[nickm]

Strange! Aren't we already including windows.h ?

[teor]

Replying to nickm:

Strange! Aren't we already including windows.h ?

MinGW's WinBase.h doesn't contain SecureZeroMemory.
(Their other memory setting functions #define down to memset, so this is probably just as well.)

We'll need to AC_CHECK_FUNC for SecureZeroMemory in configure, like in #7419. (Do we have to include windows.h explicitly, or will configure pick it up automatically?)

[nickm]

Watch out: AC_CHECK_FUNC doesn't check whether the function is declared at all; it just checks whether it's linkable. We might also need AC_CHECK_FUNC_DECL. Either way we should probably include windows.h.

[teor]

The function is defined inline in the header, there's no linker symbol for it. So we need AC_CHECK_FUNC_DECL and *not* AC_CHECK_FUNC.

[nickm]

I've tried to fix it with 1d6dd288e1c084a5118785899cca910e8c69fbb1; let's see if that works