Add getters / setters for the X509_STORE_CTX and X509_STORE functions

We only add setters for X509_STORE function pointers except for the
verify callback function.  The thought is that the function pointers
in X509_STORE_CTX are a cache for the X509_STORE functions.
Therefore, it's preferable if the user makes the changes in X509_STORE
before X509_STORE_CTX_init is called, and otherwise use the verify
callback to override any results from OpenSSL's internal
calculations.

Reviewed-by: Rich Salz <rsalz@openssl.org>