Switch to libbsd-overlay

This is more portable than libbsd as everything Just Works, even on BSD systems, and is the recommended method of consuming libbsd nowadays.

It also helpfully lets things work with glibc-provided functions for new enough glibc.

Closes: !973 (closed)

Co-authored-by: Guillem Jover guillem@hadrons.org

Signed-off-by: Sam James sam@gentoo.org