Handle infinite certificate validation loops caused by OpenSSL bug #3090.

Handle infinite certificate validation loops caused by OpenSSL bug #3090.

If OpenSSL is stuck in a validation loop, Squid breaks the loop and
triggers a
new custom SQUID_X509_V_ERR_INFINITE_VALIDATION SSL validation error.
That
error cannot be bypassed using sslproxy_cert_error because to break the
loop
Squid has to tell OpenSSL that the certificate is invalid, which
terminates
the SSL connection.

Validation loops exceeding SQUID_CERT_VALIDATION_ITERATION_MAX
iterations
are deemed infinite. That macro is defined to be 16384, but that default
can
be overwritten using CPPFLAGS.

  This is a Measurement Factory project