Delete temporary enc file if it exists
* src/common.cpp (DeleteFileUtf8): New. (CreateFileUtf8): SetLastError to avoid having it for conversions. Do not share delete to avoid problems with attachments. (get_tmp_outfile): Try to delete existing files. Fix switch to attachment.dat. Add error message on failure.
This fixes the issues related to empty messages cannot
be encrypted because the temporary file could not be created.
- GnuPG-Bug-Id: T5926