Bug 12571 - Add ability to customize SIP2 screen messages

Bug 12571 - Add ability to customize SIP2 screen messages

We should add the ability to apply a regular expression to screen
messages for the SIP2 server. This would allow libraries to not only
customize the screen messages the patron sees, but can also allow screen
messages to be translated.

Test Plan:
1) Apply this patch
2) Inspect etc/SIPconfig.xml, note the new screen_msg_regex tags
   that can be nested inside a given login tag.
3) Add one or more screen_msg_regex tags to your own SIP config
   Recommendation: s/Greetings from Koha./Welcome to your library!/g
4) Restart your SIP2 server
5) Test with a SIP2 machine, or use /misc/sip_cli_emulator.pl
6) Note your new AF fields!

Signed-off-by: Jason Burds <jburds@dubuque.lib.ia.us>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>