shim: user switcher process management struct

This change moves all user switcher and shim management to the shim package and withholds output while shim is alive. This also eliminated all exit scenarios where revert is skipped.

Signed-off-by: Ophestra Umiker <cat@ophivana.moe>

internal/app/system.go

@@ -22,6 +22,8 @@ type appSealSys struct {
 	// target user sealed from config
 	user *user.User
 
+	needRevert bool
+	saveState  bool
 	*system.I
 
 	// protected by upstream mutex