babeld: fix redistribution of ipv4 protocols.

For example, if you were saying "rip" or "ospf", the behavior
was cleary to take their v6 alternatives, namely "ripng" and
"ospf6", since their name begins the same.
babeld: Use "IS_ENABLE" macro; fix mtu printing.
babeld: remove useless variable.
babeld: remove inappropriate comments.

The local module is not present in quagga.
babeld: remove useless "const".
babeld: remove useless function (format_bool).
Fix parsing of compressed IPv4 Updates

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.