gatherstats.pl: Add '-l' option.

Add '-l' option to check newsgroup names against a list of valid
newsgroups read from a file (each group on one line, ignoring
everything after the first whitespace and so accepting files in
checkgroups format as well as (parts of) an INN active file).
New ReadGroupList().
Change ListNewsgroups() accordingly.
Amend documentation accodingly.

See #17.

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