dhcpd: remember and record hostnames; optimize get_option

dhcpd: remember and record hostnames; optimize get_option
dumpleases: show hostnames

function                                             old     new   delta
add_lease                                            230     292     +62
send_offer                                           403     421     +18
send_ACK                                             232     249     +17
read_leases                                          249     258      +9
dumpleases_main                                      604     609      +5
nobody_responds_to_arp                                84      86      +2
udhcp_end_option                                      32      30      -2
udhcp_get_option                                     222     171     -51
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(add/remove: 0/0 grow/shrink: 6/2 up/down: 113/-53)            Total: 60 bytes