httpd: add -u user[:grp] support

libbb/safe_strncpy.c

@@ -15,6 +15,7 @@
 /* Like strncpy but make sure the resulting string is always 0 terminated. */
 char * safe_strncpy(char *dst, const char *src, size_t size)
 {
-	dst[size-1] = '\0';
-	return strncpy(dst, src, size-1);
+	if (!size) return dst;
+	dst[--size] = '\0';
+	return strncpy(dst, src, size);
 }