Add tls support to capabilities exchange

To upgrade a connection to TLS or not, that is the question. It
is possible for us to send a CER offering both NO_INBAND_SECURITY
and TLS and for the peer to answer likewise: RFC 3588 doesn't make
clear that a CEA should be unambiguous about the choice of security.
Thus, if TLS is offered then assume the server is prepared to
for a handshake. Similarly, when receiving a CER, choose TLS if
it's offered and be unambiguous about our choice in CEA. There is
no ssl:maybe_accept that would let us receive a handshake if it
comes or another message if it doesn't.

The choice of TLS should probably be made into a callback so that
an application can decide based on the peer's Origin-Realm for
example. Such a callback could also be used to reject a CER/CEA.

Handle Inband-Security-Id values other than NO_INBAND_SECURITY and
TLS by assuming that they require no intervention by the transport
module, treating them like NO_INBAND_SECURITY. Whether or not this
is reasonable (or useful) is unclear. There may be a need for more
sychronization than we have on offer. (Having to do something before
taking the connection up for example.)

Note that diameter_peer_fsm must be upgraded before diameter_capx
because of the new return value from diameter_capx:recv_CEA/2.

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