New feature: Set advmss on IPv4 default route

This ensures that TCP connections terminated through the CLAT gets a sensible
TCP MSS value negotiated. By default, the value is set to the MTU of the
defaultroute minus 40. There should be no reason to change this in the vast
majority of cases, but it can be overridden with the "v4-defaultroute-advmss"
setting.

README.pod

@@ -326,6 +326,17 @@ If you know that the IPv6 Path MTU between the host and the PLAT is larger
 than 1280, you may increase this, but then you should also recompile TAYGA
 with a larger B<ipv6_offlink_mtu> setting in I<conffile.c>.
 
+=item B<v4-defaultroute-advmss=integer> (default: B<v4-defaultroute-mtu> - 40)
+
+The "advmss" value assigned to the the default route potining to the CLAT. This
+controls the advertised TCP MSS value for TCP connections made through the
+CLAT.
+
+You should normally not need to set this. By default the value is calculated by
+taking the value of B<v4-defaultroute-mtu> and substracting 40 (20 bytes for
+the IPv4 header + 20 bytes for the TCP header). If B<v4-defaultroute-mtu> is
+unset or 0, there is no default.
+
 =back
 
 =head1 LIMITATIONS

clatd

@@ -40,6 +40,7 @@ $CFG{"v4-defaultroute-enable"} = 1;	# add a v4 defaultroute via the CLAT?
 $CFG{"v4-defaultroute-replace"} = 0;	# replace existing v4 defaultroute?
 $CFG{"v4-defaultroute-metric"} = 2048;	# metric for the IPv4 defaultroute
 $CFG{"v4-defaultroute-mtu"} = 1260;	# MTU for the IPv4 defaultroute
+$CFG{"v4-defaultroute-advmss"} = 0;	# TCP MSS for the IPv4 defaultroute
 
 
 #
@@ -868,7 +869,11 @@ if(cfgbool("v4-defaultroute-enable")) {
     push(@cmdline, ("metric", cfgint("v4-defaultroute-metric")))
   }
   if(cfgint("v4-defaultroute-mtu")) {
-    push(@cmdline, ("mtu", cfgint("v4-defaultroute-mtu")))
+    push(@cmdline, ("mtu", cfgint("v4-defaultroute-mtu")));
+    push(@cmdline, ("advmss", cfgint("v4-defaultroute-advmss") ||
+                              cfgint("v4-defaultroute-mtu") - 40));
+  } elsif(cfgint("v4-defaultroute-advmss")) {
+    push(@cmdline, ("advmss", cfgint("v4-defaultroute-advmss")));
   }
   p("Adding IPv4 default route via the CLAT");
   cmd(\&err, cfg("cmd-ip"), @cmdline);