There is a function ossl_verify_cb() that fetches the custom callback
Proc from X509_STORE/X509_STORE_CTX and calls it, but it was not very
useful for SSL code. It's only used in ossl_x509store.c and ossl_ssl.c
so move X509::Store specific code to ossl_x509store.c.

Also make struct ossl_verify_cb_args and ossl_call_verify_cb_proc()
local to ossl.c.

Set verify_mode to OpenSSL::SSL::VERIFY_PEER directly. They are tests
for verify_callback so they don't need to use SSLContext#set_params.

If a client sets this to true and enables SNI with SSLSocket#hostname=,
the hostname verification on the server certificate is performed
automatically during the handshake using
OpenSSL::SSL.verify_certificate_identity().

Currently an user who wants to do the hostname verification needs to
call SSLSocket#post_connection_check explicitly after the TLS connection
is established.

This commit also enables the option in SSLContext::DEFAULT_PARAMS.
Applications using SSLContext#set_params may be affected by this.

[GH ruby#8]