implement wcsftime function

src/time/wcsftime.c

+#include <wchar.h>
+#include <time.h>
+#include <string.h>
+
+size_t wcsftime(wchar_t *wcs, size_t n, const wchar_t *f, const struct tm *tm)
+{
+	size_t k, n0=n;
+	char out[100], in[4];
+	while (*f) {
+		if (!n) return 0;
+		if (*f != '%') {
+			*wcs++ = *f++;
+			n--;
+			continue;
+		}
+		in[2] = in[3] = 0;
+		in[0] = *f++;
+		if (strchr("EO", (in[1]=*f++)))
+			in[2] = *f++;
+		k = strftime(out, sizeof out, in, tm);
+		if (!k) return 0;
+		k = mbsrtowcs(wcs, (const char *[]){out}, n, 0);
+		if (k==(size_t)-1) return 0;
+		wcs += k;
+		n -= k;
+	}
+	if (!n) return 0;
+	*wcs++ = 0;
+	return n0-n;
+}
+
+