[PATCH] Authentication support for pserver

I've patched git-cvsserver so that it supports pserver authentication as
opposed to only anonymous access. The patch itself works but I haven't
updated the relevant documentation or test cases and won't be able to do
so until after the weekend so I'm submitting what I have for review on
the list.

The pserver user/password database is stored in the config file for each
repository because I didn't know where else to put it, for example:

avar@oe:/tmp/fleh$ cat /tmp/git-test.git/config 
[core]
	repositoryformatversion = 0
	filemode = true
	bare = true

[gitcvs]
    enabled = 1

[gitcvs.users]
    avar = foobar
    whee = whoo

The user can then login/checkout and do other operations with the
user/password set in the config file:

avar@oe:/tmp/fleh$ cvs -d:pserver:whee:whoo@localhost/tmp/git-test.git login
Logging in to :pserver:whee@localhost:2401/tmp/git-test.git
avar@oe:/tmp/fleh$ cvs -d:pserver:whee:oops@localhost/tmp/git-test.git login
Logging in to :pserver:whee@localhost:2401/tmp/git-test.git
The password supplied for user whee was incorrect
cvs login: authorization failed: server localhost rejected access to
    /tmp/git-test.git for user whee

$ cvs -d:pserver:whee:whoo@localhost/tmp/git-test.git co master
cvs checkout: Updating master
U master/.emacs
[...]