SecondaryTableController: force the MSS to match pmtu on TCP SYN

Without this change, the VPN sets up a tun/ppp that needs a small
MTU, and during TCP SYN the MSS will end up matching the outgoing iface
MTU which is potentially too big.
This leads to connection flakiness. The wrong MSS is visible by
tcpdump-ing on the tun/ppp device.

With this change, the MSS now is correct.
It requires the kernel to be configured with
 CONFIG_NETFILTER_XT_TARGET_TCPMSS=y
If kernel is not configured, it silently fails.

Bug: 11579326
Change-Id: I254d8c39435b92dff91931e461e1efb8b35f6b1e