knot-*: set .meta.mainProgram

I believe it's clear what "main program" is in these cases.
It seems unclear if this change is useful, but it's very simple
and it shouldn't hurt anything.

pkgs/servers/dns/knot-dns/default.nix

@@ -81,5 +81,6 @@ stdenv.mkDerivation rec {
     license = licenses.gpl3Plus;
     platforms = platforms.unix;
     maintainers = [ maintainers.vcunat ];
+    mainProgram = "knotd";
   };
 }

pkgs/servers/dns/knot-resolver/default.nix

@@ -112,6 +112,7 @@ unwrapped = stdenv.mkDerivation rec {
     license = licenses.gpl3Plus;
     platforms = platforms.unix;
     maintainers = [ maintainers.vcunat /* upstream developer */ ];
+    mainProgram = "kresd";
   };
 };
 
@@ -126,6 +127,7 @@ wrapped-full = runCommand unwrapped.name
     ];
     preferLocalBuild = true;
     allowSubstitutes = false;
+    inherit (unwrapped) meta;
   }
   ''
     mkdir -p "$out"/bin