earlier version ok sashan@, changes since then: drop the change to the
manual, and reorder struct

ok tb@

struct bgpd_config includes various pointers and those should not be passed.
Instead use an zeroed stack object and copy_config() to ensure that all
pointers are NULL before passing the struct.
Also implement imsg_recv_config() which does the reverse.

Reported by Shibo, Shawn, Hugo, Systopia Team
OK tb@

Reflects kern_pledge.c r1.346 and r1.348.

Also in open(2): Document when __pledge_open appeared and add a missing word.

ok deraadt