ifplugd: restore auto-ifup unless -a; make iff method less iffy :D

ifplugd: restore auto-ifup unless -a; make iff method less iffy :D

function                                             old     new   delta
up_iface                                               -     112    +112
network_ioctl                                         13      38     +25
detect_link_iff                                       58      71     +13
detect_link                                          143     152      +9
ifplugd_main                                        1107    1109      +2
detect_link_wlan                                     131     125      -6
detect_link_ethtool                                   71      65      -6
detect_link_priv                                      88      80      -8
detect_link_mii                                       88      80      -8
maybe_up_new_iface                                   144      27    -117
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 4/5 up/down: 161/-145)           Total: 16 bytes

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