[crypto] Guard against mutation of certificate chain during validation

Certificate chains are reference-counted structures, and both the TLS
connection and the validator hold a reference to the same certificate
chain while validation is in progress.  The TLS connection will not
mutate the chain during this time: if a second (illegal) Certificate
record were to arrive then it would drop its reference to the existing
chain (leaving the validator as the sole possessor) before creating a
new chain to hold the received certificates.

The validator currently holds two pointers that could be invalidated
if a future code change were to cause the chain to be externally
mutatated while validation is in progress:

  - a pointer to the current certificate (for OCSP or cross-signed
    downloads) that does not hold its own reference and relies upon
    the chain's reference to keep the certificate pointer valid

  - a pointer to the current link within the certificate chain

Guard against this class of potential future code changes by promoting
the certificate pointer to hold its own reference to the certificate
so that it is guaranteed to remain valid, deleting the stored pointer
to the current link completely, and adding a function x509_link() that
is used to locate the certificate link by traversing the chain.

Signed-off-by: Michael Brown <mcb30@ipxe.org>