Implement Option --error-markers=<begin>,<end> 

* This option can be used to mark the begin/end of errors in textual
output mode, to facilitate searching/extracting errors in output files
mixing valgrind errors with program output.

* Use the new option in various existing regtests to test the various
  possible usage.



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14714 a5019735-40e9-0310-863c-91ae7b9d1cf9