core: add LUOSession= unit setting

Acquiring a LUO session from /dev/liveupdate requires privileges,
and also the device is a single-owner driver so only a single
process can open it at any given time.

Add a LUOSession= service settings that allows units running
without privileges to get a session assigned to them.

The kernel imposes a 64 chars limit on session names, which is
too short to avoid clashes, so derive a hash from joining the
unit name with the parameter name, that way two units using
the same setting don't clash.

test/units/TEST-91-LIVEUPDATE.sh

@@ -60,6 +60,17 @@ RemainAfterExit=yes
 FileDescriptorStoreMax=${maxfd}
 FileDescriptorStorePreserve=yes
 ExecStart=${cmd}
+EOF
+
+    # Create a unit with LUOSession= to test server-managed session lifecycle
+    cat >"/run/systemd/system/TEST-91-LIVEUPDATE-session.service" <<EOF
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+FileDescriptorStoreMax=10
+FileDescriptorStorePreserve=yes
+LUOSession=test-session-1 test-session-2
+ExecStart=/usr/lib/systemd/tests/unit-tests/manual/test-luo check-sessions test-session-1 test-session-2
 EOF
 }
 
@@ -226,6 +237,16 @@ EOF
           /run/TEST-91-LIVEUPDATE-failure.attempt \
           /run/TEST-91-LIVEUPDATE-failure.preserve-broken
     systemctl daemon-reload
+
+    # Verify the LUOSession= sessions was restored. Starting the unit again must
+    # reuse the restored sessions rather than handing out fresh ones (the names are
+    # already present in the fd store), and the payload verifies they are valid.
+    n_fds=$(systemctl show -P NFileDescriptorStore TEST-91-LIVEUPDATE-session.service)
+    echo "Session unit fd store count after kexec: $n_fds"
+    test "$n_fds" -eq 2
+    systemctl start TEST-91-LIVEUPDATE-session.service
+    n_fds=$(systemctl show -P NFileDescriptorStore TEST-91-LIVEUPDATE-session.service)
+    test "$n_fds" -eq 2
 else
     # Create memfds with known content and push them to our fd store.
     # Also request a LUO session, store a memfd in it, and push the session fd to the fd store.
@@ -285,6 +306,12 @@ EOF
         test "$n_fds" -eq 3
     done
 
+    # Test LUOSession= setting: start the unit and verify sessions are in its fd store
+    systemctl start TEST-91-LIVEUPDATE-session.service
+    n_fds=$(systemctl show -P NFileDescriptorStore TEST-91-LIVEUPDATE-session.service)
+    echo "Session unit fd store count: $n_fds"
+    test "$n_fds" -eq 2  # test-session-1 and test-session-2
+
     # 'systemctl kexec' auto-loads the default boot entry (i.e. the booted UKI,
     # via EFI LoaderEntrySelected/LoaderEntryDefault). Append a marker to the
     # kernel command line so we can tell the two boots apart, and also the current