irclog core module: rename old logs when switching from dir to file

irclog core module: rename old logs when switching from dir to file

It may happen that a user changes from an irclog.filename_format where
some components are files to a format where they are directories (e.g.
from '%%{where}' to '%%{where}/%Y') or conversely. In this case, we
rename the existing file/dir by appending '.old.atime' to it (atime is
the actual file/dir access time).