main: print relative path in the init success message

... but only if the relative path does not start with "..".

Makes the message easier to grasp. Example:

  $ gocryptfs -init -scryptn=10 v0.9
  [...]
  The filesystem has been created successfully.

Before:
  You can now mount it using: gocryptfs /home/jakob/src/github.com/rfjakob/gocryptfs/integration_tests/example_filesystems/v0.9 MOUNTPOINT

After:
  You can now mount it using: gocryptfs v0.9 MOUNTPOINT

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