This prevents a DoS from the local interface, which could be performed
by any user on the system running babeld.

A malicious attacker could connect to the local interface, and then
wait without reading any data.  When the send buffer eventually fills
up, babeld would block; as a result, it would become completely
inoperative.

Note that 'local_server_socket' is already non-blocking, but the
sockets spawned by accept() don't inherit this flag.  From accept(2):

  On Linux, the new socket returned by accept() does not inherit file
  status flags such as O_NONBLOCK and O_ASYNC from the listening socket.

The various cases of success or failure are quite intricate.  Since we
plan to intricate it further in an upcoming commit, it's better to
refactor it now.

We were using the Flags field instead of the AE field when parsing
compressed IPv4 prefixes in Update messages.  This bug shouldn't
trigger, since we never use compression with IPv4.

This avoids multiple config file entries for the same interface
causing an assertion failure.  Reported by Baptiste Jonglez.

Conflicts:
	CHANGES

It turns out that the rp_filter is only disabled if both all/rp_filter
and ifname/rp_filter are set to 0.  This sets and restores all of these
values explicitly, which fixes mysterious issues on distributions that
set rp_filter by default.

This avoids a crash when an interface is added both in the configuration
file and on the command-line.

This was causing the parser to ignore the rest of the file after an
unknown option.

This avoids various unpleasant edge-cases.

This probably ensures that everything that can be done from the
command-line can be done from the configuration file.

This also adds options protocol-port and protocol-group.