Explicitly pass in an SSL_CTX * to the functions that operate on one,

Explicitly pass in an SSL_CTX * to the functions that operate on one,
instead of assuming that they should use the one associated with the TLS
context. This allows these functions to be used with the additional
SSL contexts that are needed to support server-side SNI.

Also rename tls_configure_keypair() to tls_configure_ssl_keypair(), so that
these functions have a common prefix.

ok reyk@