tests: Replace calls to dd with native writeZeroes
reverse: Make uniqueIV configurable in expert mode
reverse: Filesystem is read-only of uniqueIV is enabled

Writing to the ciphertext files can rewrite the header. This
would mean we had to re-encrypt the whole file with the new IV.

This could be made more fine-grained, for example allowing
writes to everywhere but the header. However, this is
something that needs a lot of testing to ensure correctness.
Writing to the ciphertext is a niche use case of the niche
use case of using reverse mode, so it is unlikely it would
get the test coverage it needs.

To be safe, we deny all modifications of the ciphertext with
read-only filesystem error (EROFS) if uniqueIV is enabled.

Reverse mode with uniqueIV disabled still supports writing,
if somebody really needs it. This use case is not covered
by the test suite at the moment.
encfs_symlink: Fix argument naming (was reversed)

It is symlink(target, link_name), see man 3 symlink
tests: reverse: symlink absolute path inside the plaintext dir

This test currently fails because of a bug in EncFS
Remove buggy prefix check from plainPath

In reverse mode, this caused symlinks pointing to the absolute
plaintext directory to be stripped. This is what the test in
commit

	tests: reverse: symlink absolute path inside the plaintext dir

checks for.

Ignoring encfsctl, plainPath() is only called from encfs.cpp, in
_do_readlink() and _do_getattr(). Both functions get the path passed in from
FUSE. Paths from FUSE are always anchored at the mountpoint (they start with
"/", and "/" means the root of the mount). This suggests that the check can
never trigger - I have verified that it does not trigger when running the
test suite.

With this patch, the full test suite passes.
Add comments to path-handling functions in DirNode.cpp
Replace ternary operators in cipherPathWithoutRoot with if clause

Adds a few lines but makes clear what is happening.
tests: Unset ENCFS6_CONFIG before testing

This prevents unexpected failures when you have set that variable.

Also, give Test::More the number of tests that will be run for
more informative output.
Clarify read-only handling of reverse mounts without uniqueIV

Also, delete unused define

Both issues spottet by Valient Gough's review
Merge pull request #36 from rfjakob/master

Reverse mode improvements