bgpd: Ability to add/update unique extended communities

bgpd: Ability to add/update unique extended communities

Certain extended communities cannot be repeated. An example is the
BGP link bandwidth extended community. Enhance the extended community
add function to ensure uniqueness, if requested.

Note: This commit does not change the lack of uniqueness for any of
the already-supported extended communities. Many of them such as the
BGP route target can obviously be present multiple times. Others like
the Router's MAC should most probably be present only once. The portions
of the code which add these may already be structured such that duplicates
do not arise.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>