Fix non-transitive extended community handling.

Fix non-transitive extended community handling.
First of all the detection logic was totally wrong. Then filter out
non-transitive extended communities when received from an ebgp peer.
Also cleanup the type handling of ext-communities. Mainly to not have
to handle the transitive vs non-transitive versions the type is masked
with EXT_COMMUNITY_VALUE before doing the switch case for the various
types.
With this my test using ext-communities works.
OK tb@