ssl: move TLS ext dissect code to packet-ssl-utils.c

This moves the code used to parse the TLS extension into packet-
ssl-utils.c and adds an architecture that this code could be used by
dtls in the next patch. This patch should not change anything in the
functionality.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
dtls: use generic ssl dissect code for TLS extensions

This makes the DTLS dissector use the now common TLS extensions
dissector from packet-ssl-utils.c instead of its own.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>