main: fix shadow detection logic
main: fix shadow detection logic

This fired incorrectly:

	Mountpoint "/home/testuser" would shadow cipherdir "/home/testuser.cipher", this is not supported

davesteele/gocryptfs,rfjakob/gocryptfs,davesteele/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs,davesteele/gocryptfs