This was the last of the three places that do TLS from clients (logs,
control, derp). With this, iOS should be able to use the
memory-efficient x509 root CertPool.
The autoselection should pick sensible paths for all of:
- Windows (LocalAppData)
- Mac (Library/Caches)
- Unix user (XDG_CACHE_DIR)
- Linux systemd service (CACHE_DIRECTORY)
As a last resort, if cache dir lookup fails, plops sufficiently
uniquely named files into the current working directory.
Signed-off-by: David Anderson <dave@natulte.net>