Document the current semantics of the 'quiet' command prefix
in the rc.subr(8).
Fix dhclient rc.d script: it should not call err() for
non-DHCP-enabled interface when it is called from devd, because the
latter just blindly calls 'service dhclient quietstart' on each "link
up" event.
Since the 'quietstart' will silence the message "Cannot 'start' <foo>.
Set <foo>_enable to YES in /etc/rc.conf or use 'onestart' instead of
'start'." and running dhclient on the non-DHCP-enabled interface is
the same thing as running the service <foo> without <foo>_enable set,
such modification is in sync with the current semantics of the 'quiet'
prefix.
Approved by: glebius
Reviewed by: freebsd-rc list
MFC after: 2 weeks
0113cca483