udhcp: fix oversized packet sending (introduced by "slack for bad dhcp servers" options);

udhcp: fix oversized packet sending (introduced by "slack for bad dhcp servers" options);
slight optimizations and function renaming

udhcp_send_raw_packet                                  -     391    +391
udhcp_send_kernel_packet                               -     197    +197
udhcp_recv_packet                                      -     134    +134
get_raw_packet                                       353     326     -27
udhcp_get_packet                                     134       -    -134
udhcp_kernel_packet                                  197       -    -197
udhcp_raw_packet                                     391       -    -391
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(add/remove: 3/3 grow/shrink: 0/1 up/down: 722/-749)          Total: -27 bytes