Don't compile extra TCP modules by default

[jtl]

Summary

At least for now, we don't want to compile extra TCP modules by default. However, we do want to compile them as part of test builds (LINT or tinderbox).

After this change, you will need to add WITH_EXTRA_TCP_STACKS=1 to /etc/make.conf or your kernel config in order to build the extra TCP modules.

Test Plan

• mk buildkernel with the GENERIC kernel does not compile the extra module.
• mk buildkernel with the LINT kernel does compile the extra module.
• Adding makeoptions WITH_EXTRA_TCP_STACKS=1 to the kernel config causes the build to compile the extra module.

[gnn]

I'm still OK with this.