ssl,dtls: move ServerHello to ssl-utils

Changes to ServerHello:

 - Get rid of session parameter as the SslDecryptSession already
   provides a reference to SslSession. Done for the new
   ssl_dissect_hnd_srv_hello and ssl_dissect_hnd_hello_ext functions.
 - No need to generate the keyring here, decryption is only needed after
   ChangeCipherSpec and the keyring will be generated there provided
   that there are enough params (cipher, version, master-secret, etc.)
 - Get rid of labels and goto in favor of if/else.
 - DTLS: SSL_HAVE_SESSION_KEY is implicitly set when
   ssl_generate_keyring_material succeeds, no need to set it. Remove it.
 - Remove `if (tree || ssl)` since expert info seems not to apply
   otherwise (this also needs changes in common and handshake
   dissection). (Expert info from the TLS extensions, not the
   ServerHello itself.)

Other changes:

 - ClientHello: renamed some fields shared with ServerHello.

Change-Id: I466b905d990489e03250bac97bf9d862ff82ce6c
Reviewed-on: https://code.wireshark.org/review/3020
Reviewed-by: Evan Huus <eapache@gmail.com>