RFC: need one IFCAP bit for NETMAP

hi,
in order to import netmap support into the tree (see
http://info.iet.unipi.it/~luigi/netmap ), i need
to use one of the if_cap{abilities|enable} bits to report
that the interface supports this mode and record whether
the mode is enabled or not.

Bit  0x100000 is currently the first unused bit so if
there are no objections i will add the following
to sys/net/if.h

#define IFCAP_NETMAP    0x100000 /* netmap mode supported/enabled */

and the following change to sbin/ifconfig/ifconfig.c

-"\21VLAN_HWFILTER\23VLAN_HWTSO\24LINKSTATE"
+"\21VLAN_HWFILTER\23VLAN_HWTSO\24LINKSTATE\25NETMAP"

That's all you need, as netmap mode is not supposed to be
set from sbin/ifconfig.

cheers
luigi