Simplify extra_certs handling w.r.t. assign_privkey()

Simplify extra_certs handling w.r.t. assign_privkey()

With the free_supporting_certs[] array, there's no need to pass
extra_certs[] in to the GnuTLS 2 version of assign_privkey() just to
let it remove the certs that it wants to steal. Instead, remove them as
we *find* them and put them into the supporting_certs[] array, and mark
them as "to be freed".

By keeping the free_my_certs[] array, this also fixes a bug in the
GnuTLS 2 code where we would end up freeing a cert which was obtained
by gnutls_certificate_get_issuer(), which we are *supposed* to treat as
being constant.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>