local switch: Implement shared-secret feature

This allows for a “shared secret” to be entered for a network switch,
segmenting traffic so that multiple people could use the feature
simultaneously without accidentally entering into or interfering with
each other's networks.

Takes a string specified in the configuration file (using the
net_%02i_secret key) and hashes it through SHA3-256 to prepend to each
data packet.  This hash is used to compare packets on reception and
allow or discard them.

src/include/86box/network.h

@@ -98,6 +98,7 @@ typedef struct netcard_conf_t {
     char     host_dev_name[128];
     uint32_t link_state;
     uint8_t  switch_group;
+    char     secret[256];
     uint8_t  promisc_mode;
     char     slirp_net[16];
     char     nrs_hostname[128];