cvsserver: anonymous cvs via pserver support

git-cvsserver now knows how to do the pserver auth chat when the user
is anonymous. To get it to work, add a line to your inetd.conf like

cvspserver stream tcp nowait nobody git-cvsserver pserver

(On some inetd implementations you may have to put the pserver parameter twice.)

Commits are blocked. Naively, git-cvsserver assumes non-malicious users. Please
review the code before setting this up on an internet-accessible server.

NOTE: the <nobody> user above will need write access to the .git directory
to maintain the sqlite database. Updating of the sqlite database should be
put in an update hook to avoid this problem, so that it is maintained by
users with write access.