bgpd: Make socket init separate, so unit tests work again.
* Separate out BGP socket initialisation from bgp_get, and make it an
explicit function. Allows unit tests to work again and probably also
benefits dry-run.
* bgpd.c: (bgp_get) move socket init out...
(bgp_socket_init) to here
* bgp_main.c: and call it after dry-run.
* bgpd.h: (bgp_socket_init) add prototype
