udhcpc6: set -x options in request

udhcpc6: set -x options in request

Last foru commits:

function                                             old     new   delta
option_to_env                                        621     791    +170
.rodata                                           168351  168505    +154
attach_option                                        431     506     +75
add_d6_client_options                                112     167     +55
d6_option_strings                                     30      84     +54
udhcp_str2optset                                     644     660     +16
d6_optflags                                           12      20      +8
udhcpc6_main                                        2590    2596      +6
udhcpc_main                                         2648    2651      +3
read_optset                                           15      18      +3
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 10/0 up/down: 544/0)            Total: 544 bytes

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