PR23226: Added ability to run sample scripts with typing whole path

PR23226: Added ability to run sample scripts with typing whole path

* cmdline.h: Added new command line option '--example'
* cmdline.cxx: Added new command line option '--example'
* session.h: Added new flag 'run_example' to track example option
* session.cxx: Updated constructors to initialize new flag and added
  logic to set run_example flag in parse_cmdline()
* main.cxx: If '--example' is specified then search for script within
  example directory and run it if one hit is found
* man/stap.1.in: Added new entry for '--example' option
* testsuite/buildko/example.stp: New file to test new option
* testsuite/buildok/example.stp: New file to test new option