systemd: Allow setting the transient hostname via DHCP

This permits using method_set_hostname but still denies
method_set_static_hostname. As a result DHCP clients can now always set
the transient hostname via the SetHostname method of the D-Bus interface
of systemd-hostnamed (org.freedesktop.hostname1.set-hostname).
If the NixOS option networking.hostName is set to an empty string (or
"localhost") the static hostname (kernel.hostname but NOT /etc/hostname)
will additionally be updated (this is intended).

From "man hostnamectl": The transient hostname is a fallback value
received from network configuration. If a static hostname is set, and is
valid (something other than localhost), then the transient hostname is
not used.

Fix #74847.

Note: It's possible to restrict access to the org.freedesktop.hostname1
interface using Polkit rules.

pkgs/os-specific/linux/systemd/0007-hostnamed-localed-timedated-disable-methods-that-cha.patch

@@ -14,16 +14,6 @@ diff --git a/src/hostname/hostnamed.c b/src/hostname/hostnamed.c
 index 21f6471495..8c5af7619f 100644
 --- a/src/hostname/hostnamed.c
 +++ b/src/hostname/hostnamed.c
-@@ -422,6 +422,9 @@ static int method_set_hostname(sd_bus_message *m, void *userdata, sd_bus_error *
-         if (r < 0)
-                 return r;
- 
-+        return sd_bus_error_setf(error, SD_BUS_ERROR_NOT_SUPPORTED,
-+            "Changing system settings via systemd is not supported on NixOS.");
-+
-         if (isempty(name))
-                 name = c->data[PROP_STATIC_HOSTNAME];
- 
 @@ -478,6 +481,9 @@ static int method_set_static_hostname(sd_bus_message *m, void *userdata, sd_bus_
          if (r < 0)
                  return r;