Move error checks for packet dropping

struct general_hdr is not general enough for all types of NWP
packets; only the first two fields are necessary. Therefore, the
error-checking code for the number of HIDs and the hardware
address length should be moved to more local code for
announcements and neighbor lists.
Implement efficient monitoring algorithm

Every struct hid_dev now has a timer to periodically begin a
monitoring interval. It is also necessary to allocate memory
for a temporary target for each monitoring interval.

There are also bools for whether monitoring is currently
occuring, for determining whether contact has been made with
any neighbor during the current monitoring interval (to
diagnose network partitions), and for whether the neighbor has
yet been remonitored if failure is suspected.

Upon arrival of a neighbor list, the monitoring algorithm may
begin in process_neigh_list(). monitor() initiates a monitoring
interval, using pick_random_neigh() to select a random target
to monitor.

To accomplish this, the monitoring algorithm first pings the
selected target, which may result in a timer expiration and
callback of end_ping_failure(). In that case, investigative
neighbors may be used to verify that the given target has
failed. If, at the end of the investigative time period, no
response from the target has been received, a timer expiry
callbacks into end_interval_failure(), which may declare a
node as failed.

The forward declaration of end_interval_failure() is necessary
because of the cyclic nature between monitor(),
end_ping_failure(), and end_interval_failure().

In order to send monitoring packets, send_monitoring() is
implemented. Note that const u8 *inv should not be used
for Ping and Ack packets, and in those cases should be set to
NULL.

A function update_neigh() is added to update the status of a
hardware address for which we don't have a struct hrdw_addr
reference.
Enable receipt and processing of monitoring packets

nwp_rcv() may now accept monitoring packets and distribute
them to process_monitoring(), which updates clocks, changes
timers, and dispatches packets as necessary depending on the
type of packet that has been received.

Since monitoring packets may now be received, it is possible
for monitoring to be successful (the target responds). Hence,
end_interval_success() is added.
Add functionality for cleaning neighbor list

NWP monitoring has the power to detect and declare interfaces and
neighboring HIDs as failed. To do this, clean_neigh_list() is
periodically called by a timer in each hdev to clean the neighbor
list of any failed nodes by deleting them.
Fix spelling of announcement

Fixed all spelling mistakes for "announcement."