Use synchronous IO on file handles created in remove_all NT6 implementation.

This eliminates spinning while iterating over directory contents as by default
file handles created by NtCreateFile are non-blocking. Because of this
NtQueryDirectoryFile didn't block and returned STATUS_PENDING without actually
updating the iteration state. Eventually this would cause directory iteration
to fail with a hard error.

src/operations.cpp

@@ -2004,7 +2004,7 @@ uintmax_t remove_all_nt6_by_handle(HANDLE h, path const& p, error_code* ec)
                     FILE_LIST_DIRECTORY | DELETE | FILE_READ_ATTRIBUTES | FILE_WRITE_ATTRIBUTES | SYNCHRONIZE,
                     FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE,
                     FILE_OPEN,
-                    FILE_OPEN_FOR_BACKUP_INTENT | FILE_OPEN_REPARSE_POINT
+                    FILE_SYNCHRONOUS_IO_NONALERT | FILE_OPEN_FOR_BACKUP_INTENT | FILE_OPEN_REPARSE_POINT
                 );
 
                 if (!NT_SUCCESS(status))

src/windows_tools.hpp

@@ -156,6 +156,9 @@ struct object_attributes
 #ifndef FILE_DIRECTORY_FILE
 #define FILE_DIRECTORY_FILE 0x00000001
 #endif
+#ifndef FILE_SYNCHRONOUS_IO_NONALERT
+#define FILE_SYNCHRONOUS_IO_NONALERT 0x00000020
+#endif
 #ifndef FILE_OPEN_FOR_BACKUP_INTENT
 #define FILE_OPEN_FOR_BACKUP_INTENT 0x00004000
 #endif