cmd/sharefs: implement shared filesystem

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

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

options.nix

@@ -40,6 +40,40 @@ in
         '';
       };
 
+      sharefs = {
+        user = mkOption {
+          type = types.str;
+          default = "sharefs";
+          description = ''
+            Name of the user to run the sharefs daemon as.
+          '';
+        };
+
+        group = mkOption {
+          type = types.str;
+          default = "sharefs";
+          description = ''
+            Name of the group to run the sharefs daemon as.
+          '';
+        };
+
+        name = mkOption {
+          type = types.str;
+          default = "/sdcard";
+          description = ''
+            Host path to mount sharefs on.
+          '';
+        };
+
+        source = mkOption {
+          type = types.nullOr types.str;
+          default = null;
+          description = ''
+            Writable backing directory. Setting this to null disables sharefs.
+          '';
+        };
+      };
+
       apps = mkOption {
         type =
           let