Complete certificate chains using external intermediate certificates

Complete certificate chains using external intermediate certificates
stored in sslproxy_foreign_intermediate_certs file.

Many origin servers do not send complete certificate chains. Many
browsers use certificate extensions in the server certificate to
download the missing intermediate certificates automatically from
the Internet. Squid does not do that (yet?).

This patch adds the sslproxy_foreign_intermediate_certs configuration directive
to allow an admin to supply a file with intermediate certificates that
Squid may use to complete certificate chains. These intermediate
certificates are _not_ treated as trusted root certificates.

This is a Measurement Factory project.