[tls] Treat session secret as "resumption master secret"

When resuming a session in TLS versions 1.2 and earlier, the master
secret is simply reused.  The value stored in the session is therefore
the same as the master secret used in the connection.

For TLS version 1.3, there is a separate concept of a "resumption
master secret" that is derived from the original connection's master
secret, and from which the resumed connection's new master secret will
be derived.

Rename the session master_secret to resumption_master_secret to
clarify this separation.

Resume use of the master secret (i.e. copy the secret from the session
to the connection) only after the cipher suite has been selected, to
reduce differences with the expected flow for TLS version 1.3.

Signed-off-by: Michael Brown <mcb30@ipxe.org>