nmeter: convert field list to bit list

nmeter: convert field list to bit list

function                                             old     new   delta
rdval                                                 34     157    +123
collect_int                                          123     122      -1
collect_thread_nr                                     65      62      -3
collect_blk                                          559     552      -7
collect_if                                           207     199      -8
collect_fork                                         119     111      -8
collect_ctx                                          119     111      -8
collect_fd                                            81      71     -10
collect_swp                                          120     107     -13
collect_cpu                                          623     610     -13
collect_mem                                          371     339     -32
rdval_loadavg                                         38       -     -38
vrdval                                               170       -    -170
------------------------------------------------------------------------------
(add/remove: 0/2 grow/shrink: 1/10 up/down: 123/-311)        Total: -188 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>