cmd/sharefs: implement shared filesystem

This is for passing files between applications, similar to android /sdcard.

Signed-off-by: Ophestra <cat@gensokyo.uk>

flake.nix

@@ -69,6 +69,8 @@
             withRace = true;
           };
 
+          sharefs = callPackage ./cmd/sharefs/test { inherit system self; };
+
           hpkg = callPackage ./cmd/hpkg/test { inherit system self; };
 
           formatting = runCommandLocal "check-formatting" { nativeBuildInputs = [ nixfmt-rfc-style ]; } ''