The function sntp_set_servers() duplicates the strings supplied in the
server_url[] array into new strings but forgets to allocate the extra
byte needed for the \0 terminator for each string.
Fix the problem by using strdup(), which allocates the right amount of
memory and copies the string at once.
This reworks the accounting of the time to address the wrapping of the
rtc counter. It currently assumes that the time is accessed more
frequently than the counter would wrap. It also adds a semaphore to
synchronize access to the time storage.
