git-cvsserver: clarify directory list

The documentation and error messages for git-cvsserver include some
references to a "whitelist" that is not otherwise included in the
documentation. When different parts of the documentation do not use
common language, this can lead to confusion as to how things are meant
to operate.

Further, the word "whitelist" has cultural implications that make its
use non-inclusive. Thankfully, we can remove it while increasing
clarity.

Update Documentation/git-cvsserver.txt in a similar way to the previous
change to Documentation/git-daemon.txt. The optional '<directory>...'
list can specify a list of allowed directories. We refer to that list
directly inside of the documentation for the GIT_CVSSERVER_ROOT
environment variable.

While modifying this documentation, update the environment variables to
use a list format. We use the modern way of tabbing the description of
each variable in this section. We do _not_ update the description of
'<directory>...' to use tabs this way since the rest of the items in the
OPTIONS list do not use this modern formatting.

A single error message in the actual git-cvsserver.perl code refers to
the whitelist during argument parsing. Instead, refer to the directory
list that has been clarified in the documentation.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>