cvsserver: Limit config parser to needed options

cvsserver: Limit config parser to needed options

Change the configuration parser so that it ignores
everything except for ^gitcvs.((ext|pserver).)?
This greatly reduces the risk of failing while
parsing some unknown and irrelevant config option.

The bug that triggered this change was that the
parsing doesn't handle sections that have a
subsection and a variable with the same name.

While this bug still remains, all remaining
causes can be attributed to user error, since
there are no defined variables gitcvs.ext and
gitcvs.pserver.

Signed-off-by: Frank Lichtenheld <frank@lichtenheld.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>