Simplify BGP unnumbered configuration by eliminating the unessential.

Simplify BGP unnumbered configuration by eliminating the unessential.

To make BGP configuration as simple as possible, assume the capability
extended-nexthop to be default for interface neighbors. Also allow the
ability to specify remote-as on the same line as neighbor interface to
make BGP unnumbered configuration a single line.

One corner case. This is the first feature for which the default for a
member is different from the default for a peer-group. Since advertising
the capability is only done for interface neighbors, the capability is
not set for the peer-group, but is automatically set for interface
neighbors that belong to that peer-group. So, if you want to disable the
advertisement of this capability for an interface neighbor, you must
do it per each interface neighbor.

The patch is more complicated than it needs to be due to the handling
of quagga reload and appropriate updates to the show running output.

Ticket: CM-11830
Reviewed By: CCR-4958
Testing Done: Usual coterie, including manual

(cherry picked from commit 347914a0a785993200cb04ae3cbf8ff9a9104d60)