They aren't meant to be critical (uncatchable) errors.
Tested with nix-env + checkMeta:
[ "x86_64-linux" "i686-linux" "x86_64-darwin" "aarch64-linux" ]
The motivation for this change is the following: As gnu-netcat,
e. g. does not support ipv6, it is not suitable as default netcat.
This commit also fixes all obvious build issues caused by this change.