Improve handling for disallowed filenames
* src/common.cpp (get_tmp_outfile): Use a generic name after 50 tries.
If we could not create the file after 50 tries, maybe
the filename is invalid. In that case we now use a
generic "attachment" name. That name is visible in
the "Save As" dialog, but in the OOM the name
is set according to the original filename.
- GnuPG-Bug-Id: T4835