ps: conditionally support additional -o FIELDs

ps: conditionally support additional -o FIELDs

function                                             old     new   delta
procps_scan                                         1409    1642    +233
out_spec                                             220     300     +80
func_ruser                                             -      36     +36
func_rgroup                                            -      36     +36
func_group                                            13      49     +36
func_nice                                              -      29     +29
buffer_fill_and_print                                179     196     +17
send_tree                                            355     360      +5
mkfs_vfat_main                                      1604    1609      +5
display_speed                                         85      90      +5
scriptreplay_main                                    194     197      +3
find_out_spec                                         55      58      +3
changepath                                           192     195      +3
sha1_process_block64                                 497     484     -13
------------------------------------------------------------------------------
(add/remove: 3/0 grow/shrink: 10/1 up/down: 491/-13)          Total: 478 bytes

Signed-off-by: David Krakov <krakov@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>