Bug 7787: Make the SIP server much more robust.

Bug 7787: Make the SIP server much more robust.

Be liberal in what we accept, but strict in what we send:
Never exit the server process, but send a SC_RESEND message (96)
to the client if we received anything we don't understand.
 This is consistent with SIP server implementations of other ILSs.

Signed-off-by: Colin Campbell <colin.campbell@ptfs-europe.com>
Signed-off-by: Ian Walls <koha.sekjal@gmail.com>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>