[core] no graceful-restart-bg on OpenBSD, NetBSD
[core] no graceful-restart-bg on OpenBSD, NetBSD

disable server.graceful-restart-bg on OpenBSD and NetBSD

kqueue is not inherited across fork, and OpenBSD and NetBSD do not
implement rfork() (implemented on FreeBSD and DragonFly)

lighttpd has not implemented rebuilding the kqueues after fork,
so server.graceful-restart-bg is disabled on OpenBSD and NetBSD.

gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4