Add root store directory fallback on Linux/Unix.

Add root store directory fallback on Linux/Unix.

When reading the root store directory for the first time, if
the read produced no data and the SSL_CERT_DIR environment
variable wasn't set, see if /etc/ssl/certs gives a different answer.

This change also changes the LastWriteTime model for caching
to not pin the symlink target on the first read, and support the
bundle file being a symlink (and the target being updated to
trigger refresh).