[Feature Request] Add an option to unmount

[mhogomchungu]

When mounting a file system, gocryptfs seems to do the following:-

1. Use "mount" command.
2. If the above fail then use fusermount3 command if present.
3. If the above is absent then use fusermount command.
4. If the above is absent then error out and report fusermount command not found.

Unmounting is left to the user and i do not think there is a way of knowing which command was use at mounting. Both fusermount3 and fusermount report a file system as "fuse.gocryptfs" for example.