app/instance: wrap internal implementation

This reduces the scope of the fst package, which was growing questionably large.

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

internal/app/instance/shim.go

@@ -0,0 +1,6 @@
+package instance
+
+import "git.gensokyo.uk/security/fortify/internal/app/internal/setuid"
+
+// ShimMain is the main function of the shim process and runs as the unconstrained target user.
+func ShimMain() { setuid.ShimMain() }