Improve IPsec config

- IKEv2 mode uses retransmit-timeout instead of dpdtimeout.
- Increase timeouts from 120s to 300s, so that the VPN server can
  keep the VPN connection open if the client's network is unstable.

docs/ikev2-howto-zh.md

@@ -823,7 +823,7 @@ To customize IKEv2 or client options, run this script without arguments.
      rightrsasigkey=%cert
      narrowing=yes
      dpddelay=30
-     dpdtimeout=120
+     retransmit-timeout=300s
      dpdaction=clear
      auto=add
      ikev2=insist

docs/ikev2-howto.md

@@ -825,7 +825,7 @@ View example steps for manually configuring IKEv2 with Libreswan.
      rightrsasigkey=%cert
      narrowing=yes
      dpddelay=30
-     dpdtimeout=120
+     retransmit-timeout=300s
      dpdaction=clear
      auto=add
      ikev2=insist

extras/ikev2setup.sh

@@ -1107,7 +1107,7 @@ conn ikev2-cp
   rightrsasigkey=%cert
   narrowing=yes
   dpddelay=30
-  dpdtimeout=120
+  retransmit-timeout=300s
   dpdaction=clear
   auto=add
   ikev2=insist

vpnsetup_alpine.sh

@@ -323,7 +323,7 @@ conn shared
   rekey=no
   keyingtries=5
   dpddelay=30
-  dpdtimeout=120
+  dpdtimeout=300
   dpdaction=clear
   ikev2=never
   ike=aes256-sha2,aes128-sha2,aes256-sha1,aes128-sha1,aes256-sha2;modp1024,aes128-sha1;modp1024

vpnsetup_amzn.sh

@@ -325,7 +325,7 @@ conn shared
   rekey=no
   keyingtries=5
   dpddelay=30
-  dpdtimeout=120
+  dpdtimeout=300
   dpdaction=clear
   ikev2=never
   ike=aes256-sha2,aes128-sha2,aes256-sha1,aes128-sha1,aes256-sha2;modp1024,aes128-sha1;modp1024

vpnsetup_centos.sh

@@ -407,7 +407,7 @@ conn shared
   rekey=no
   keyingtries=5
   dpddelay=30
-  dpdtimeout=120
+  dpdtimeout=300
   dpdaction=clear
   ikev2=never
   ike=aes256-sha2,aes128-sha2,aes256-sha1,aes128-sha1,aes256-sha2;modp1024,aes128-sha1;modp1024

vpnsetup_ubuntu.sh

@@ -384,7 +384,7 @@ conn shared
   rekey=no
   keyingtries=5
   dpddelay=30
-  dpdtimeout=120
+  dpdtimeout=300
   dpdaction=clear
   ikev2=never
   ike=aes256-sha2,aes128-sha2,aes256-sha1,aes128-sha1,aes256-sha2;modp1024,aes128-sha1;modp1024