kbx: Add extra flags to fopen for use by Windows.

* kbx/keybox-search.c (open_file): Use sysopen and sequential.
* kbx/keybox-update.c (create_tmp_file): Ditto.
(blob_filecopy): Ditto.
(keybox_set_flags): Ditto.
(keybox_delete): Ditto.
(keybox_compress): Ditto.

Under Windows "sysopen" requests that direct API calls (CreateFile et
al.) are used instead of the libc wrappers. This may or may not
improve the performance.

Using "sequential" is a hint to Windows to assume that a file is in
general access in a sequential manner. This will have an affect only
with a future libgpg-error.