cvsserver: Add some useful commandline options

cvsserver: Add some useful commandline options

Make git-cvsserver understand some options inspired by
git-daemon, namely --base-path, --export-all, --strict-paths.

Also allow the caller to specify a whitelist of allowed
directories, again similar to git-daemon.

While already adding option parsing also support the common
--help and --version options.

Rationale:

While the gitcvs.enabled configuration option already
offers means to limit git-cvsserver access to a repository,
there are some use cases where other methods of access
control prove to be more useful.

E.g. if setting up a pserver for a collection of public
repositories one might want limit the exported repositories
to exactly the directory this collection is located whithout
having to worry about other repositories that might lie around
with the configuration variable set (never trust your users ;)

Signed-off-by: Frank Lichtenheld <frank@lichtenheld.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>