bunsen dejagnu parsing: tolerate misordered .log <-> .sum

bunsen dejagnu parsing: tolerate misordered .log <-> .sum

As keiths & serhei have long ago discovered and we rediscovered now,

       make -j check

type dejagnu runs can regularly scramble .sum / .log file segments so
they no longer parallel.  One offender is gcc's
contrib/dg-extract-results.sh that assembles .log/.sum files from
parallel-executed pieces .... and proceeds to SORT rows within .sum
file .exp segments (only).  Why?  Why?  WHYYYYYY?!  y tho

That resulted in many logfile cursor = NULL results.  Tweak
i-dejagnu-parser to tolerate this case better by pre-parsing segments
of the .exp file loglines.  The logic here may break if some wacky
tool reorders the sequence of .exp files, but this has not been
observed.