ssl,dtls: simplify keyfile handling

Previously, the keylog file would be fully parsed when an encrypted
pre-master secret is encountered or in the ChangeCipherSpec stage. There
was also a lot of duplication in the key logfile parsing.

This patch simplifies the key logfile parsing by using regular
expressions. Rather than scanning the key logfile for a specific key,
do this scan once at ssl init and save the results to a hashtable. The
map for session ID/tickets to master keys already existed, another one
for client random to master key and encrypted pre-master to pre-master
was added. This could later also be wired to the "Export SSL Keys"
menu item for improved reliability (when no session ID or tickets are
available, the client random could be used).

The ssl_{save,restore}_session{,_ticket} functions have been converted
to a single function that looks up a key (sid / client random / encr.
pre-master) to a (pre-)master secret.

Other minor changes: return booleans for some functions that can only
fail/pass. Remove some functions from the ssl-utils header that have
become private a few commits ago. Remove some outstanding issues
from the comments in packet-ssl as they are already done, add myself
to the ssl-utils header.

These changes pass the test suite and the sample Session Ticket-enabled
capture from https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=5963

On-the-fly decryption are broken with this patch since keylog files are
read once at the start of a capture. This will be solved in a future
patch.

Change-Id: Idb343abe161950b5f3ff61bee093d0f4ef9655bd
Reviewed-on: https://code.wireshark.org/review/3057
Reviewed-by: Evan Huus <eapache@gmail.com>
Petri-Dish: Evan Huus <eapache@gmail.com>
Petri-Dish: Alexis La Goutte <alexis.lagoutte@gmail.com>
Reviewed-by: Michael Mann <mmann78@netscape.net>