libbb: add strncpy_IFNAMSIZ

libbb: add strncpy_IFNAMSIZ

function                                             old     new   delta
...
udhcp_read_interface                                 225     220      -5
brctl_main                                          1151    1146      -5
add_interface                                        109     104      -5
ipaddr_list_or_flush                                2174    2167      -7
do_add_ioctl                                          88      80      -8
vconfig_main                                         249     240      -9
do_del_ioctl                                          78      68     -10
do_iplink                                           1186    1173     -13
do_if_fetch                                          766     753     -13
buffer_fill_and_print                                196     179     -17
parse_args                                          1709    1684     -25
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/20 up/down: 11/-140)         Total: -129 bytes