monkey patch more POSIX-like behavior out of winsock 2

Windows sockets functions look on the outside like they behave similarly
to POSIX functions, but there are many subtle and glaring differences,
including errors reported via WSAGetLastError, read, write, and close do
not work on sockets, setsockopt takes a (char *) rather than (void *),
etc.

This header implements wrappers that coerce more POSIX-like behavior
from these functions, making portable code easier to develop.

BENEFITS:
One does not necessarily need to sprinkle #ifdefs around code to handle
the Windows and non-Windows behavior when porting code.

CAVEATS:
There may be performance implications with the 'mother-may-I'
approach to determining if a descriptor is a socket or a file.

The errno mappings are not 100% what one might expect compared to POSIX
since there were not always good 1:1 equivalents from the WSA errors.

include/string.h

@@ -50,4 +50,20 @@ void * memmem(const void *big, size_t big_len, const void *little,
 	size_t little_len);
 #endif
 
+#ifdef _WIN32
+#include <errno.h>
+
+static inline char  *
+posix_strerror(int errnum)
+{
+	if (errnum == ECONNREFUSED) {
+		return "Connection refused";
+	}
+	return strerror(errnum);
+}
+
+#define strerror(errnum) posix_strerror(errnum)
+
+#endif
+
 #endif