svcj: correctly handle kernels without INET or INET6

If either INET or INET6 is not enabled in the kernel, then the jail(8)
options ip4=<new|inherit> resp. ip6=<new|inherit> are not available.
Detect this case and don't try to provide those options, otherwise
svcjs will not start.

Do this automatically (without a warning) so that net_basic, which
includes both netv4 and netv6, continues to work as expected.

If _svcj_ipaddrs is explicitly configured with an address for an IP
version not supported by the kernel, issue a warning but continue to
start the service. This can result in the service being started with
fewer addresses than expected, but never more.

Reviewed by: netchild, des
Approved by: des (mentor)
Differential Revision: https://reviews.freebsd.org/D49976