ipfilter: Disable ipfs(8) by default

At the moment ipfs(8) is a tool that can be easily abused. Though the
concept is sound the implementation needs some work.

ipfs(8) should be considered experimental at the moment.

This commit also makes ipfs support in the kernel optional.

Reviewed by: emaste, glebius
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D53787