privsep: Only set RLIMIT_NOFILES for supported platforms

privsep: Only set RLIMIT_NOFILES for supported platforms

Basically only for NetBSD, DragonFlyBSD, kqueue or epoll platforms.
All others will use poll(2) and return EINVAL if nfds > RLIMIT_NOFILES.

Trying to work out our nfs limit relies on already listening to
all FD's which may not be the case and was problematic.
Luckily all platforms aside from Solaris have new file creation
sandboxed away aside from accept(2) so this should still be safe.