Merge branch 'am/trace2-with-valueless-true'

The trace2 code was not prepared to show a configuration variable
that is set to true using the valueless true syntax, which has been
corrected.

* am/trace2-with-valueless-true:
  trace2: prevent segfault on config collection with valueless true

diff --cc t/t0210-trace2-normal.sh
Simple merge

diff --cc trace2.c
Simple merge

diff --cc trace2/tr2_tgt_event.c
Simple merge

diff --cc trace2/tr2_tgt_perf.c
Simple merge