- Move the code to generate test report to the test tool(in this
   case mysqltest) where the best control of what failed is
- Simplify the code in mysql-test-run.pl
- mysqltest will now find what diff to use in a best effort attempt
  using "diff -u", "diff -c" and finally dumping the two files verbatim
  in the report.


client/mysqltest.c:
  - Add new option --tail-lines indicating how many lines of the result to output
    when generating report for a failure
  - Remove eval_result, noone knows what it should do and it's not used.
  - Add support for best effort execution of systems builtin "diff",
    try with "diff -u" and "diff -c" and if that fails dump the whole content
     of teh two files to report
  - Use one common function when comapring two files, used when test 
    has completed and the result should be compared to the .result file
    as well as using it from myqltest's builtin diff_files command.
  - Output the last lines og the result so far  in the report that mysqltest
    generates in case of a test failure.
mysql-test/lib/mtr_report.pl:
  - Remove code for generating diff between .reject and .result file,
    that is handled by mysqltest(or the test tool) from now on.
  - Add better comments
mysql-test/mysql-test-run.pl:
  - Remove the --udiff flag to mysql-test-run.pl, if the systems diff supports
    unified diff it will be used automatically
  - Remove the code for "mtr_show_failed_diff", the report from mysqltest
    will know include the diff so if mysqltest(or another test tool)
    fails, just display it's report what went wrong
  - Pass --tail-lines=20 to mysqltest to it will shos the last 20 lines 
    from the result in the report in case of failure.	
mysql-test/r/mysqltest.result:
  Update result file now when the output from mysqltest has been sent to /dev/null
mysql-test/t/mysqltest.test:
  - Improve tests for --diff_files
  - Remove temporary files in var/tmp generated in the fly
  - Send output from test for --diff_files to /deb/null since only the error
    code it returns are of interest and tyhe outpu may vary depending
    on what builtin diff is being used.