add support for RCS keyword expansion over pserver protocol

We can simply ask the CVS server to expand keywords for us, instead
of forcing binary file mode with the -kb option. The CVS repository
contains per-file keyword expansion defaults the server will use.
Files checked out by cvsclient.py should now match what a regular
CVS client would check out by default.

Add test cases which verify that we create the same snapshot ID
for a repository which uses the Id keyword in a file, regardless
of whether this repository is accessed via rsync or pserver.