telnetd: we only need CONFIG_UNIX98_PTYS from the Linux kernel

(re-sending this as it got ignored completey and the format of the
previous mail was probably not correct - please let me know if there's
anything else I can do to get this trivial fix applied)

for telnetd to work, we only need CONFIG_UNIX98_PTYS to be enabled
in the Linux kernel - DEVPTS_FS has been obsolete for some time

Signed-off-by: Martin Kaiser <martin@kaiser.cx>