Add $ssl_verify_partial_chains option for OpenSSL. (closes #3916)

Add $ssl_verify_partial_chains option for OpenSSL.  (closes #3916)

The reworked OpenSSL certificate validation took away a "feature" of
the previous implementation: the ability to reject a node in the chain
and yet continue to the next node.

If this new option is set to 'yes', enables OpenSSL's
X509_V_FLAG_PARTIAL_CHAIN flag to reinstate the functionality and permit
to use a non-root certificate as the trust anchor.

This option is only available if OpenSSL offers the
X509_V_FLAG_PARTIAL_CHAIN macro, which should be the case as of 1.0.2b
or later.

Code written by Kevin McCarthy and Matthias Andree.