Cast mode constants to mode_t in case it is signed

Cast mode constants to mode_t in case it is signed

At least one compiler (Borland) defines mode_t as just "short" which is
signed.  This breaks code like

  switch(archive_entry_filetype(e)) {
    case AE_IFREG:
    ...
  }

if AE_IFREG and other constants have a longer signed type (int) because
sign extension of the mode_t return type from archive_entry_filetype
changes its value.  Avoid the problem by ensuring the type of the
constants matches mode_t.

SVN-Revision: 4128