Fix #369 relative command path

The docs say:

    The command can be either absolute (e.g. /path/to/programname) or relative (e.g. programname). 
    If it is relative, the supervisord’s environment $PATH will be searched for the executable.

This fixes relative path like to/programname as stated in #369 .
If the documentation says  `If it is relative, the supervisord’s environment $PATH will be searched for the executable.`, but this code assumes the program is relative to the configuration file directory, which is a very different thing.  It introduces new behaviour that does not match the documentation.