contrib/mklog.py: Add main function

contrib/mklog.py: Add main function

This adds a main() function to mklog.py (like e.g. check_GNU_style.py
has), which makes it easier to import and invoke from another python
script.  This is useful when using a wrapper script to set up the python
environment.

Smoke tested by using the modified mklog.py to generate the ChangeLog
for this patch.

contrib/ChangeLog:

	* mklog.py (main): New.