Merge all files:group sections into one, where key is name of the group
and value is source name, optionally followed by destination path.
This breaks old mkit.ini's, but significantly simplifies future ones.
1. Only one section is needed,
2. group list is implied from the files section, removing the need
of lists:group,
3. destination path has been made optional (by default, basename of
the source path is used).