Remove the "disable IPv6" build option.

Remove the "disable IPv6" build option.

For quite a while --enable-ipv6 and its CMake equivalent have been the
default; one effect of --disable-ipv6 (and its CMake equivalent) was
enabling the "gateway" primitive, but this is no longer the case, other
effects are:

* disabling the [self-implemented] IPv6 address string parsing,
* ignoring AF_INET6 results from getaddrinfo(3), and
* making "link proto \ip6" an invalid syntax.

Other than that, after many developments in the 25 years since commit
7fe3c11 building "without" IPv6 no longer manages any external
dependencies, so it is time to defragment this solution space.  Remove
all INET6 conditionals from the C source code and the tests, remove the
build options from Autoconf and CMake, in the build matrix scripts
replace IPV6 with PROTOCHAIN.