BGP: Extend 'next hop keep' and 'next hop self' options

Extend 'next hop keep' and 'next hop self' options to have boolean values
(enabled / disabled) and also values 'ibgp'/ 'ebgp' to restrict it to
routes received from IBGP / EBGP. This allows to have it enabled by
default in some cases, matches features of other implementations, and
allows to handle some strange cases like EBGP border router with 'next
hop self' also doing IBGP route reflecting.

Change default of 'next hop keep' to enabled for route servers, and
'ibgp' for route reflectors.

Update documentation for these options.