Initial support for encrypted filenames

To avoid leaking which services/documents one has access to, one would
like to encrypt filenames in the store. Using base64 encoding on
symmetrically encrypted names is short enough for filenames.
Optimisation/preparations for filename encryption

We're now loading all the encrypted filenames in a key/value store upon
start for the commands we need it for. Future work will include reusing
matching folder names, as every time you encrypt a string it's
different. This is good if we don't want to disclose we have the same
names in different folders, but bad for reusing the existing folder
structure.

Also preliminary support for inserting a password with an encrypted
path.
All operations now use encrypted filenames

If $PREFIX/.fnsympass.gpg exists, all operations automatically use
encrypted filenames. Paths that exist will be reused. If a clear
path/filename exists, it will also be used as to retain backwards
compatibility.

Names are still exposed via history files, so an interactive mode should
be introduced.
File descriptors unncessary, support for editing
Proceed regardless of passphrase availability

If we can't decrypt filenames, show the store tree anyway