core: copy the host's os-release for /run/host/os-release

Currently for portable services we automatically add a bind mount
os-release -> /run/host/os-release. This becomes problematic for the
soft-reboot case, as it's likely that portable services will be configured
to survive it, and thus would forever keep a reference to the old host's
os-release, which would be a problem because it becomes outdated, and also
it stops the old rootfs from being garbage collected.

Create a copy when the manager starts under /run/systemd/propagate instead,
and bind mount that for all services using RootDirectory=/RootImage=, so
that on soft-reboot the content gets updated (without creating a new file,
so the existing bind mounts will see the new content too).

This expands the /run/host/os-release protocol to more services, but I
think that's a nice thing to have too.

Closes https://github.com/systemd/systemd/issues/28023

test/units/testsuite-50.sh

@@ -583,4 +583,6 @@ grep -q -F "MARKER_CONFEXT_123" /etc/testfile
 systemd-confext unmerge
 rm -rf /run/confexts/ testjob/
 
+systemd-run -P -p RootImage="${image}.raw" cat /run/host/os-release | cmp "${os_release}"
+
 touch /testok

test/units/testsuite-82.sh

@@ -54,6 +54,8 @@ elif [ -f /run/testsuite82.touch2 ]; then
     # Test that we really are in the new overlayfs root fs
     read -r x </lower
     test "$x" = "miep"
+    cmp /etc/os-release /run/systemd/propagate/os-release
+    grep -q MARKER=1 /etc/os-release
 
     # Switch back to the original root, away from the overlayfs
     mount --bind /original-root /run/nextroot
@@ -92,7 +94,16 @@ elif [ -f /run/testsuite82.touch ]; then
     mkdir -p /run/nextroot /tmp/nextroot-lower /original-root
     mount -t tmpfs tmpfs /tmp/nextroot-lower
     echo miep >/tmp/nextroot-lower/lower
-    mount -t overlay nextroot /run/nextroot -o lowerdir=/:/tmp/nextroot-lower,ro
+
+    # Copy os-release away, so that we can manipulate it and check that it is updated in the propagate
+    # directory across soft reboots.
+    mkdir -p /tmp/nextroot-lower/usr/lib
+    cp /etc/os-release /tmp/nextroot-lower/usr/lib/os-release
+    echo MARKER=1 >>/tmp/nextroot-lower/usr/lib/os-release
+    cmp /etc/os-release /run/systemd/propagate/os-release
+    (! grep -q MARKER=1 /etc/os-release)
+
+    mount -t overlay nextroot /run/nextroot -o lowerdir=/tmp/nextroot-lower:/,ro
 
     # Bind our current root into the target so that we later can return to it
     mount --bind / /run/nextroot/original-root