[TCP]: Do not present confusing congestion control options by default.

Create TCP_CONG_ADVANCED option, akin to IP_ADVANCED_ROUTER, which
when disabled will bypass all of the congestion control Kconfig
options and leave the user with a safe default.

That safe default is currently BIC-TCP with new Reno as a fallback.

Signed-off-by: David S. Miller <davem@davemloft.net>