ifupdown: support 'link' address family

ifupdown: support 'link' address family

Does not configure anything. L2 configuration hook scripts should do
their job on receiving ADDRFAM=link. Configuration will be done only
once, irrespective of L3 protocols used.

Using the 'link' family in the interfaces file conforms to the Debian
implementation:

http://sources.debian.net/src/ifupdown/0.7.47.1/link.defn

function                                             old     new   delta
link_methods                                           -      12     +12
addr_link                                              -      12     +12
link_up_down                                           -       6      +6
static.addr_fams                                      12      16      +4
------------------------------------------------------------------------------
(add/remove: 3/0 grow/shrink: 1/0 up/down: 34/0)               Total: 34 bytes

Signed-off-by: Kaarle Ritvanen <kaarle.ritvanen@datakunkku.fi>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>