Change interpretation of --checkgroups to template

In most hierarchies, the list of valid newsgroups will
change over time, so you'll have to use another
checkgroups file for each month. gatherstats will now
understand the value of --checkgroups to be a template
and amend it with each month it is processing.

Documentation changed accordingly.

Signed-off-by: Thomas Hochstein <thh@inter.net>