doc/prerst2man.py: Convert execfile to import

doc/prerst2man.py: Convert execfile to import

excefile is gone in Python 3 [1].  Instead of exec-ing the
configuration, it's easier to insert the source directory in Python's
path [2], and just import the configuration.  With this change,
prerst2man.py is compatible with both Python 2 and 3.

[1]: https://docs.python.org/3.0/whatsnew/3.0.html#builtins
[2]: https://docs.python.org/3/library/sys.html#sys.path