- convert a few xopen3(,,0) into xopen(,). Also peruse the fact that xopen defaults...

- convert a few xopen3(,,0) into xopen(,). Also peruse the fact that xopen defaults to 0777 on it's own, so we don't need to xopen3(,,0777).
  Saves a few bytes:
$ size busybox.old busybox                                text    data     bss     dec     hex filename
 839676    8780  243592 1092048  10a9d0 busybox.old
 839660    8780  243592 1092032  10a9c0 busybox
$ make CC=gcc-4.2-HEAD bloatcheck
function                                             old     new   delta
static.vconfig_main                                  281     279      -2
static.rx_main                                      1076    1074      -2
static.mt_main                                       294     292      -2
static.gzip_main                                     753     751      -2
static.mkfs_minix_main                              4034    4030      -4
static.unzip_main                                   1771    1766      -5
static.crontab_main                                 1513    1507      -6
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/7 up/down: 0/-23)             Total: -23 bytes