telnetd: some simplifications and better error hadling.

telnetd: some simplifications and better error hadling.
telnetd: don't SIGKILL child when closing the session.
kernel will seng SIGHUP for us.

static.iacs_to_send                                    -      15     +15
.rodata                                           123418  123429     +11
make_new_session                                     549     525     -24
send_iac                                              26       -     -26
free_session                                         144     118     -26
telnetd_main                                        1303    1261     -42
------------------------------------------------------------------------------
(add/remove: 1/1 grow/shrink: 1/3 up/down: 26/-118)           Total: -92 bytes
   text    data     bss     dec     hex filename
 676341    2538   12104  690983   a8b27 busybox_old
 676234    2538   12104  690876   a8abc busybox_unstripped