udhcpc[6]: do not pass xid around, keep it in client_data.xid

function                                             old     new   delta
perform_release                                      105     169     +64
perform_d6_release                                   259     262      +3
init_d6_packet                                        84      85      +1
send_d6_discover                                     286     285      -1
send_d6_select                                       128     126      -2
send_d6_renew                                        176     174      -2
send_d6_info_request                                  65      63      -2
udhcpc_main                                         2555    2551      -4
send_select                                          130     122      -8
send_renew                                            99      91      -8
send_discover                                         89      81      -8
udhcpc6_main                                        2636    2602     -34
send_release                                          74       -     -74
------------------------------------------------------------------------------
(add/remove: 0/1 grow/shrink: 3/9 up/down: 68/-143)           Total: -75 bytes

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