Shuffle the cafile= context logics to libsecurity class PeerOptions

* Rename the parameter tls-cafile= and allow multiple cafile= entries to
  be used in squid.conf.

* Document the capath= option as only supported by OpenSSL and LibreSSL.

* Add GnuTLS support for CA loading.
  Although at this point GnuTLS contexts are not yet created.

diff --cc doc/release-notes/release-4.sgml
Simple merge

diff --cc src/cf.data.pre
Simple merge