Try to detect incomplete locking of v1-encrypted directory

[ebiggers]

Unlike v2, fscrypt lock on a v1-encrypted directory doesn't warn about in-use files, as the kernel doesn't provide a way to easily detect it. In #214 this caused some confusion.

It might be possible to implement a heuristic where we check whether the filenames in the directory seem to be no-key names or not. If yes, the directory is fully locked, otherwise it's incompletely locked (or unlocked).