sysutils/serf: update to 0.10.1

While we are at it, fix build on riscv64 by bumping the
golang.org/x/{net,sys} dependencies to version new enough
to support riscv64-freebsd. Remove patches thus obsoleted.

Changelog: https://github.com/hashicorp/serf/releases/tag/v0.10.1

Approved by: swills (implicit)