cvsserver: Make always-binary mode a config file option

cvsserver: Make always-binary mode a config file option

The config option gitcvs.allbinary may be set to force all entries to
get the -kb flag.

In the future the gitattributes system will probably be a more
appropriate way of doing this, but that will easily slot in as the
entries lines sent to the CVS client now have their kopts set via the
function kopts_from_path().

In the interim it might be better to not just have a all-or-nothing
approach, but rather detect based on file extension (or file contents?).
That would slot in easily here as well.  However, I personally prefer
everything to be binary-safe, so I just switch the switch.

Signed-off-by: Andy Parkins <andyparkins@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>