Added support for removing read-only files on Windows.

Reworked remove() operation to separate POSIX and Windows implementations.
On Windows, if the file to be removed is read-only, try to reset the read-only
attribute before deleting the file. If deleting fails (other than because the
file is already deleted), try to restore the read-only attribute.

As a side effect, we were able to remove an implementation detail value from
the file_type enum that was used by the old remove() implementation.

Added a test for remove() on a read-only file on Windows. Also added tests
for remove_all(), including for cases with symlinks, hardlinks and read-only
files.

Closes #216.

doc/release_history.html

@@ -49,6 +49,7 @@ <h2>1.78.0</h2>
   <li>Optimized overloads of <code>path::assign</code>, <code>path::append</code>, <code>path::concat</code> and the corresponding operators to avoid unnecessary path copying and reduce the amount of code redundancy.</li>
   <li>On POSIX systems, fixed <code>absolute(p, base)</code> returning a path with root name <code>base.root_name()</code> if <code>p</code> starts with a root directory. In such a case <code>p</code> is already an absolute path and should be returned as is.</li>
   <li><code>create_directories</code> no longer reports an error if the input path consists entirely of dot (".") and dot-dot ("..") elements. The implementation is no longer using recursion internally and therefore is better protected from stack overflow on extremely long paths.</li>
+  <li>On Windows, <code>remove</code> now supports deleting read-only files. The operation will attempt to reset the read-only attribute prior to removal. Note that this introduces a possibility of the read-only attribute being left unset, if the operation fails and the original value of the attribute fails to be restored. This also affects <code>remove_all</code>. (<a href="https://github.com/boostorg/filesystem/issues/216">#216</a>)</li>
   <li>Fixed a linking error about unresolved references to Boost.ContainerHash functions when user's code includes <code>boost/filesystem/path.hpp</code> but not <code>boost/container_hash/hash.hpp</code> and the compiler is set to preserve unused inline functions. (<a href="https://github.com/boostorg/filesystem/issues/215">#215</a>)</li>
 </ul>
 

include/boost/filesystem/file_status.hpp

@@ -46,10 +46,8 @@ enum file_type
     fifo_file,
     socket_file,
     reparse_file, // Windows: FILE_ATTRIBUTE_REPARSE_POINT that is not a symlink
-    type_unknown, // file does exist, but isn't one of the above types or
+    type_unknown  // file does exist, but isn't one of the above types or
                   // we don't have strong enough permission to find its type
-
-    _detail_directory_symlink // internal use only; never exposed to users
 };
 
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