Evaluate commands passed to "exec" and "system" to expand any $variables before executing command.


client/mysqltest.c:
  do_exec, do_system 
   - call do_eval on the command to be executed in order to expand any $variables
   - Remove old subst_env_var and my_popen, not needed anymore
  Rewrite 'replace_strings' into 'replace_strings_append'
   - copy whole strings instead of byte by byte copy
   - insert result directly inito dynamic_string, no need to check if out 
     string needs to be realloced for every byte.
   - Add comments and DBUG_PRINT's
  New function 'fix_win_paths', detect filenames in win format that should be converted 
  do_eval
  - Only set "escaped" if next char is \ or $
mysql-test/mysql-test-run.pl:
  Always pass path for DBUG .trace file in unix format
  Add search path client_debug to find debug compiled windows binaries
  Remove unused MYSQL_TEST_WINDIR and MASTER_WINMYSOCK
mysql-test/r/mysqldump.result:
  Update test result
mysql-test/t/client_xml.test:
  Use " instead of '
mysql-test/t/mysql_client_test.test:
  Remove the useless "exec echo" command
mysql-test/t/mysqltest.test:
  Escape $variables passed to --exec, that should not be evaluated in exec.
mysql-test/t/rpl000015.test:
  Remove unneccessary replace
mysql-test/t/system_mysql_db_fix.test:
  Call the "shell script" $MYSQL_FIX_SYSTEM_TABLE using --system