ports-mgmt/parfetch-overlay: Add overlay to fetch distfiles in parallel and with pipelining

Ports that fetch multiple distfiles from the same host can
especially benefit from this.

This is a glue application between libcurl and the ports
framework. It comes with a ports overlay that replaces the
default do-fetch and checksum targets.

This is an experiment. Use at your own risk.

WWW: https://github.com/t6/parfetch
Demo: https://asciinema.org/a/ogjs0MiCNZxqwYy2xliHfYEaM

Some unscientific but repeatable numbers: tokei (152 distfiles)
fetch is down from ~44s to ~3s. wlroots (34 distfiles) fetch is
down from ~12s to ~3s. rust makesum (28 distfiles) is down from
~90s to ~50s.