The purpose of "rc-manager=symlink" is so that the administrator can point
the "/etc/resolv.conf" as a symlink to a certain file, and thus indicating
that a certain component is responsible to manage resolv.conf, while others
should stay away from it.
For example, systemd-resolved never touches "/etc/resolv.conf", but
expects the admin to setup the symlink appropriately. It also recognizes
whether the symlink points to it's own resolv.conf in /run or to another
component.

Previously, "rc-manager=symlink" would always replace a regular file
with a symlink to "/var/run/NetworkManager/resolv.conf". Only if
"/etc/resolv.conf" is already a symlink somewhere else, NM would not
touch it. This with the exception that if "/etc/resolv.conf" points to
"/var/run/NetworkManager/resolv.conf", it would replace the symlink
with the same link to raise inotify events.

Change behavior so if "/etc/resolv.conf" is already a regular file, keep
it as file.
This means, if you have multiple components that don't care, everybody
can write the "/etc/resolv.conf" (as file) and there is no clear
expressed responsibility.
It was wrong that NetworkManager would convert the file to a symlink,
this should be reserved to the admin. Instead, NetworkManager should
accept that the intent is unspecified and preserve the regular file.
It's up to the admin to replace the symlink to somewhere else (to keep
NM off), or to point it to "/var/run/NetworkManager/resolv.conf", to show
the explicit intent.

The wrong behavior causes dangling symlinks when somebody disables
NetworkManager for good.

https://bugzilla.redhat.com/show_bug.cgi?id=1367551