net-mgmt/semaphore: fix build on riscv64-freebsd

Bump golang.org/x/sys, go.etcd.io/bbolt dependencies to
versions new enough to support riscv64-freebsd.

Fix pkg-plist while we are at it.

Sponsored by: Berliner Linux User Group e.V.
Approved by: portmgr (build fix blanket)