busybox

telnetd: fix handling of short writes to pty

If a write to pty is short, remove_iacs() can be run on a buffer repeatedly.
This, for example, can eat 0xff chars (IACs, in telnet terms).

Rework the logic to handle IACs in a special "write to pty" function.

function                                             old     new   delta
telnetd_main                                        1662    1750     +88

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>