Merge from vendor branch TNF:

Merge from vendor branch TNF:
Initial import of pkgsrc-todo-1.0:

This package contains two tools for maintaining pkgsrc/doc/TODO.

add_todo adds lines for newly available updates for packages to
the TODO file.

obsolete_todo checks the contents of pkgsrc/doc/TODO against updates
noted in the CHANGES-* files and removes entries for packages which
were already updated to newer files.