sysutils/go-btfs: add new port

BitTorrent File System (BTFS) is a protocol forked from IPFS that utilizes
the TRON network and the BitTorrent Ecosystem for integration with DApps and
smart contracts.

WWW: https://www.bittorrent.com/btfs/

Reviewed by: dmgk
Differential Revision: https://reviews.freebsd.org/D24670