netcat: make netcat-openbsd the default netcat (#19411)

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.

pkgs/tools/networking/netcat/default.nix

@@ -1,17 +0,0 @@
-{ stdenv, fetchurl }:
-
-stdenv.mkDerivation {
-  name = "netcat-gnu-0.7.1";
-
-  src = fetchurl {
-    url = mirror://sourceforge/netcat/netcat-0.7.1.tar.bz2;
-    sha256 = "1frjcdkhkpzk0f84hx6hmw5l0ynpmji8vcbaxg8h5k2svyxz0nmm";
-  };
-
-  meta = with stdenv.lib; {
-    description = "Utility which reads and writes data across network connections";
-    homepage = http://netcat.sourceforge.net/;
-    license = licenses.gpl2Plus;
-    platforms = platforms.unix;
-  };
-}