feat: Add support for masking symlinks

[mmoya]

Hello

gocryptfs 2.4.0, linux, encrypted directory backed by maestral, using deterministic filenames. Creating a symlink in the plaintext side creates a symlink in the encrypted part and I get the following error:

Cannot upload symlink. Symlinks are not currently supported by the public Dropbox API.

The feature request is a mode for storing symlinks as normal files in the encrypted directory.