In 76226a34 ("ifup-eth: apply PERSISTENT_DHCLIENT to IPv6 dhclient daemon"),
one PERSISTENT_DHCLIENT option works for both IPv4 and IPv6, so the IPv4
and IPv6 settings are consistent. However, the user settings for IPv4 and IPv6
are limited.

For example, users try to adopt both IPv4 and IPv6 protocol. DHCPv6 servers
may be not stable as DHCP servers, thus the expectation for obtaining IPv6
address is lower than IPv4 address. For users, IPv4 address must be
obtained for the basic communication. So the PERSISTENT_DHCLIENT option is
set to be "yes | 1" for IPv4 address. However, the network service may be
stuck until it fails due to missing DHCPv6 servers, and the remaining devices also
cannot obtain both IPv4 and IPv6 addresses. The problem is very serious for users.

Here, I add a new PERSISTENT_DHCLIENT_IPV6 option only for IPv6 dhclient daemon,
so users can set IPv4 and IPv6 separately.

Fixes: 76226a3 ("ifup-eth: apply PERSISTENT_DHCLIENT to IPv6 dhclient daemon")
Signed-off-by: Zhiqiang Liu <liuzhiqiang26@huawei.com>

Add documentation for RES_OPTIONS.

Related: #1555001

Resolve: BZ #1691552

Now with systemd, this does not work pretty well and users really should
use systemd unit-files for this.

Add option for time saving waiting

Replace date with simple timeout

Rename configurable variable and add documentation

Print log only when didn't reach IP address in time

Add support for IPv6 address

Remove unneeded output

Change -ge to -gt, because equal value is ok

Clarify DNS name option in documentation

  This concept is quite outdated, and not sane to use at all.

  Other references to these files were removed, as well as irrelevant
  info in sysconfig.txt, and some dead code in init.d/functions file.