portable: log structured message when attach/detach succeeds

Currently portabled is completely silent (when not using debug level). But
when the system state is changed (ie: a portable is attached or detached)
there are no traces left in the journal. Log at info level when either of
those operations succeed, as they are effectively changing the state of
the system.

Create new MESSAGE_IDs for these logs, and also append PORTABLE_ROOT=
(and PORTABLE_EXTENSION= if any), like the units themselves are
configured to do via LogExtraFields=, so that the same metadata can
be found in the attach/detach messages and in logs from the units
themselves.

catalog/systemd.catalog.in

@@ -748,3 +748,23 @@ Compatibility support for System V services in systemd is deprecated. Please
 make sure to update the package in question to provide proper, native systemd
 unit files. Contact vendor if necessary. Compatibility support for System V
 services is deprecated and will be removed soon.
+
+-- 187c62eb1e7f463bb530394f52cb090f
+Subject: A Portable Service has been attached
+Defined-By: systemd
+Support: %SUPPORT_URL%
+Documentation: https://systemd.io/PORTABLE_SERVICES/
+
+A new Portable Service @PORTABLE_ROOT@ (with extensions: @PORTABLE_EXTENSION@) has
+been attached to the system and is now available for use. The list of attached
+Portable Services can be queried with 'portablectl list'.
+
+-- 76c5c754d628490d8ecba4c9d042112b
+Subject: A Portable Service has been detached
+Defined-By: systemd
+Support: %SUPPORT_URL%
+Documentation: https://systemd.io/PORTABLE_SERVICES/
+
+A Portable Service @PORTABLE_ROOT@ (with extensions: @PORTABLE_EXTENSION@) has been
+detached from the system and is no longer available for use. The list of attached
+Portable Services can be queried with 'portablectl list'.