telnetd: at Alexander Kriegisch <Alexander@kriegisch.name> insistence

telnetd: at Alexander Kriegisch <Alexander@kriegisch.name> insistence
add an option to close sessions as soon as child exits.
Maybe it should be a CONFIG option. OTOH, maybe it should be always on,
as it mimics, say, getty's behaviour.

function                                             old     new   delta
handle_sigchld                                         -      49     +49
telnetd_main                                        1312    1355     +43
.rodata                                           123429  123466     +37
packed_usage                                       22770   22806     +36
make_new_session                                     525     532      +7
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 4/0 up/down: 172/0)             Total: 172 bytes
   text    data     bss     dec     hex filename
 676285    2538   12104  690927   a8aef busybox_old
 676421    2538   12104  691063   a8b77 busybox_unstripped