Enable SSL_MODE_RELEASE_BUFFERS by default
Fix memory leak in fake cert generation code

The code in pxy_ossl_servername_cb() which generated the forged
certificates did not call SSL_CTX_free() on the newly allocated SSL_CTX
struct after associating it with the SSL struct, which increments the
reference count internally.  Also add some comments explaining OpenSSL
reference counting behaviour to be more explicit on what happens to the
instances that OpenSSL keeps track of.