dhclient(8) should not close stdio using daemon(3) function.

dhclient(8) is chrooted so opening /dev/null always will fail.
In capability world this is also annoying because we getting error that
open(2) is not permitted in Capsicum. dhclient(8) is closing stdio by
precaching fd to /dev/null before chroot.
This is done few lines below daemon(3) function so let's not try to do that
in daemon(3) function.

Reviewed by: cem@
Differential Revision: https://reviews.freebsd.org/D12826