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.

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