pcrextend: allow access to the userspace event type field when measuring something

pcrextend: allow access to the userspace event type field when measuring something

It think we should move most measurements out of the individual tools
requesting them and into the pcrextend service via Varlink, so that
fewer components require access to the TPM.

This only works however, if we can actually write full-blown event log
records via this mechanism, and for that we still were missing access to
the userspace event type we insert into the event log. Add that.