Add a bunch of SSL_xxx() functions for configuring the temporary RSA and DH

private keys and/or callback functions which directly correspond to their
SSL_CTX_xxx() counterparts but work on a per-connection basis. This is needed
for applications which have to configure certificates on a per-connection
basis (e.g. Apache+mod_ssl) instead of a per-context basis (e.g.
s_server).

For the RSA certificate situation is makes no difference, but for the DSA
certificate situation this fixes the "no shared cipher" problem where the
OpenSSL cipher selection procedure failed because the temporary keys were not
overtaken from the context and the API provided no way to reconfigure them.

The new functions now let applications reconfigure the stuff and they are in
detail: SSL_need_tmp_RSA, SSL_set_tmp_rsa, SSL_set_tmp_dh,
SSL_set_tmp_rsa_callback and SSL_set_tmp_dh_callback.  Additionally a new
non-public-API function ssl_cert_instantiate() is used as a helper function
and also to reduce code redundancy inside ssl_rsa.c.

Submitted by: Ralf S. Engelschall
Reviewed by: Ben Laurie