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