clarify usage of NETMAP_BUF
Bring in a number of new features, mostly implemented by Michio Honda:

- the VALE switch now support up to 254 destinations per switch,
  unicast or broadcast (multicast goes to all ports).

- we can attach hw interfaces and the host stack to a VALE switch,
  which means we will be able to use it more or less as a native bridge
  (minor tweaks still necessary).
  A 'vale-ctl' program is supplied in tools/tools/netmap
  to attach/detach ports the switch, and list current configuration.

- the lookup function in the VALE switch can be reassigned to
  something else, similar to the pf hooks. This will enable
  attaching the firewall, or other processing functions (e.g. in-kernel
  openvswitch) directly on the netmap port.

The internal API used by device drivers does not change.

Userspace applications should be recompiled because we
bump NETMAP_API as we now use some fields in the struct nmreq
that were previously ignored -- otherwise, data structures
are the same.

Manpages will be committed separately.
Always define INSTALL_DDIR and define it such that it contains no extra
/ characters rather than removing them later on.  This should fix
release builds.

PR:		conf/178963
Reviewed by:	gjb, hrs