mapred input and output dirs must be absolute

DFS converts relative pathnames to be under /user/$USER. But MapReduce jobs may be submitted by a different user than is running the jobtracker and tasktracker. Thus relative paths must be resolved before a job is submitted, so that only absolute paths are seen on the job tracker and tasktracker. I think the simplest way to fix this is to make JobConf.setInputDir(), setOutputDir(), etc. resolve relative pathnames.