Convert from st.m_time (second-precision time_t) to st.m_tim (struct timespec). To compensate for the potential case where a filesystem provides resolution in seconds only, always read the directory if no pack files are known to exist. Otherwise, there is a race condition when gotd repo_write creates a new pack and a request arrives for repo_read shortly after. Caught by a regression test for gotd on empty repositories. Test failure pointed out by Mikhail. ok op@

Unfortunately, MacOS doesn't provide sys/tree.h -- so the compat check for this must happen portably.

This is searched for portably.

The end goal here is to remove the dependency of repository.c on privsep.c during compilation.