dhcpcd: fix to work with systemd

Currently, dhcpcd does not work well with systemd. When using dhcpcd
to configure network, the /etc/resolv.conf contents are not correct.
This issue could easily be reproduced by using 'qemu + slirp' to
start a systemd based image and using dhcpcd to configure network.
The expected 'nameserver 10.0.2.3' is not in /etc/resolv.conf.

The root cause of this problem is that dhcpcd assumes the resolvconf
should recognize .protocol suffix[1]. But systemd's resolvconf (which
is a symlink to resolvectl) has a limited support for traditional
resolvconf interface[2], and "may not work with all clients"[3]. This
of cource includes the clients that use the .protocol suffix.

The current situation is:
1. systemd is not going to support the .protocol suffix in the foreseeable
   near future[4].
2. dhcpcd does not want to merge systemd specific patch and insists
   systemd needs to consider the .protocol suffix[5][6].

It's a normal thing that people have different opinions. As a build system
that supports such combination, however, we do need to come up with a
solution to fix this typical integration problem, making dhcpcd and systemd
work together.

This patch solves this integration problem by relying on dhcpcd's ability
to manage its own resolv.conf contents. But instead of letting it to write
to /etc/resolv.conf directly, we supply the generated contents to resolvconf.
In this way, the resolvconf still stands in the central place and dhcpcd remains
a supplier to it. And the /etc/resolv.conf can get the correct contents.
With this patch, dhcpcd could work with both sysvinit and systemd.

[1] https://man.archlinux.org/man/resolvconf.8.en
[2] https://man.archlinux.org/man/resolvectl.1#COMPATIBILITY_WITH_RESOLVCONF(8)
[3] https://wiki.archlinux.org/title/systemd-resolved
[4] https://github.com/systemd/systemd/issues/25032
[5] https://github.com/NetworkConfiguration/dhcpcd/pull/152
[6] https://github.com/NetworkConfiguration/dhcpcd/issues/146



Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
(cherry picked from commit 935ae419)
Signed-off-by: Steve Sakoman <steve@sakoman.com>