internal/outcome: optional shim private dir

This is a private work directory owned by the specific shim. Useful for sockets owned by this instance of the shim and requires no direct assistance from the priv-side process.

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

internal/outcome/dispatcher_test.go

@@ -701,6 +701,8 @@ func (panicDispatcher) stat(string) (os.FileInfo, error)                    { pa
 func (panicDispatcher) open(string) (osFile, error)                         { panic("unreachable") }
 func (panicDispatcher) readdir(string) ([]os.DirEntry, error)               { panic("unreachable") }
 func (panicDispatcher) tempdir() string                                     { panic("unreachable") }
+func (panicDispatcher) mkdir(string, os.FileMode) error                     { panic("unreachable") }
+func (panicDispatcher) removeAll(string) error                              { panic("unreachable") }
 func (panicDispatcher) exit(int)                                            { panic("unreachable") }
 func (panicDispatcher) evalSymlinks(string) (string, error)                 { panic("unreachable") }
 func (panicDispatcher) prctl(uintptr, uintptr, uintptr) error               { panic("unreachable") }