net: l2: ppp: reject unsupported options

Both ASYNC_CTRL_CHAR_MAP and MAGIC_NUMBER are not supported right
now. Send Configure-Reject for them instead of Configure-Nak, as we
don't even propose new values in reply (which should be part of
Configure-Nak).

Send also Configure-Reject for MRU option, as we don't respect it
either.

Drop both count_rej and count_nack, as we can rely solely on nack_idx to
send Configure-Reject or not.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>