e_os.h: bump minimal _WIN32_WINNT.

       /*

        * Defining _WIN32_WINNT here in e_os.h implies certain "discipline."

        * Most notably we ought to check for availability of each specific

        * routine with GetProcAddress() and/or guard NT-specific calls with

        * GetVersion() < 0x80000000. One can argue that in latter "or" case

        * we ought to /DELAYLOAD some .DLLs in order to protect ourselves

        * against run-time link errors. This doesn't seem to be necessary,

        * because it turned out that already Windows 95, first non-NT Win32

        * implementation, is equipped with at least NT 3.51 stubs, dummy

        * routines with same name, but which do nothing. Meaning that it's

        * apparently sufficient to guard "vanilla" NT calls with GetVersion

        * alone, while NT 4.0 and above interfaces ought to be linked with

        * GetProcAddress at run-time.

        * routine that was introduced after denoted _WIN32_WINNT with

        * GetProcAddress(). Normally newer functions are masked with higher

        * _WIN32_WINNT in SDK headers. So that if you wish to use them in

        * some module, you'd need to override _WIN32_WINNT definition in

        * the target module in order to "reach for" prototypes, but replace

        * calls to new functions with indirect calls. Alternatively it

        * might be possible to achieve the goal by /DELAYLOAD-ing .DLLs

        * and check for current OS version instead.

        */

#    define _WIN32_WINNT 0x0400

#    define _WIN32_WINNT 0x0501

#   endif

#   if !defined(OPENSSL_NO_SOCK) && (defined(_WIN32_WINNT) || defined(_WIN32_WCE))

       /*