For security reasons, it can be important to limit which IP addresses a
LibVNCClient-based client should listen for reverse connections. This
commit adds that option.

To preserve binary backwards-compatibility, the field was added to the end
of the rfbclient struct, and the function ListenAtTcpPort retains its
signature (but calls the new ListenAtTcpPortAndAddress).

[jes: shortened the commit subject, added a longer explanation in the
commit body and adjusted style]
Signed-off-by: Luca Stauble <gnekoz@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>