Use libbsd when available
Use libbsd when available

Use the libbsd (https://libbsd.freedesktop.org/) overlay to transparently
use the bsd compatibility layer without needing any actual code changes.

Remove now unnecessary direct access to libbsd includes. And remove
checks for unused functions.

huitema/getdns,huitema/getdns,huitema/getdns,huitema/getdns