Split out the PSK preamble from tls_process_key_exchange()
Split out the PSK preamble from tls_process_key_exchange()

The tls_process_key_exchange() function is too long. This commit starts
the process of splitting it up by moving the PSK preamble code to a
separate function.

Reviewed-by: Richard Levitte <5fb523282dd7956571c80524edc2dccfa0bd8234@openssl.org>

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl