Bug 37087: Add TCP keepalive support to SIP server

Bug 37087: Add TCP keepalive support to SIP server

This change adds the ability to enable and configure TCP keepalive
support for the SIP server using SIPconfig.xml.

For the sake of backwards compatibility, it defaults to disabled
and additional parameters default match typical kernel defaults.

Technical detail can be found in the perldoc for C4/SIP/SIPserver.pm

Test plan:
0. Apply the patch
1. koha-sip --restart kohadev
2. apt-get update && apt-get install tcpdump
3. In one window, run "tcpdump -A -n -v -i any 'port 6001'"
4. In another window, run the following:
echo -e "9300CNterm1|COterm1|CPCPL|\r" | nc 127.0.0.1 6001 -v
5. Note in tcpdump output that after the initial flood of packets,
nothing more is received

6. vi /etc/koha/sites/kohadev/SIPconfig.xml
7. In the "server-params" element, add attributes like the following:
custom_tcp_keepalive='1'
custom_tcp_keepalive_time='10'
custom_tcp_keepalive_intvl='5'
8. koha-sip --restart kohadev
9. In one window, run "tcpdump -A -n -v -i any 'port 6001'"
10. In another window, run the following:
echo -e "9300CNterm1|COterm1|CPCPL|\r" | nc 127.0.0.1 6001 -v
11. Note in tcpdump output that after the initial flood of packets,
ACK packets are sent out every 10+ seconds for the idle connection

Signed-off-by: Tadeusz „tadzik” Sośnierz <tadeusz@sosnierz.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>