Change the default build behavior so we don't compile extra TCP modules by
default. At least initially, the feature to support multiple TCP stacks is
aimed at supporting advanced use cases and TCP development, but it is not
necessarily aimed at a wide audience. Therefore, there is no need to build
and install the extra TCP stacks by default. Instead, the people who are
using or developing this functionality can add the extra option to build/
install the extra TCP stacks.

However, we do want to build the extra TCP stacks as part of test builds
(e.g. LINT or tinderbox) to ensure that developers who are testing their
changes will know that their changes do not break the additional TCP
stack modules.

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

Differential Revision: https://reviews.freebsd.org/D6795
Reviewed by: sjg
Approved by: re (kib)