git-cvsimport: add support for CVS pserver method HTTP/1.x proxying

git-cvsimport: add support for CVS pserver method HTTP/1.x proxying

This patch adds support for 'proxy' and 'proxyport' connection options
when using the pserver method for the CVS Root.

It has been tested with a Squid 2.5.x proxy server.

Quoting from the CVS info manual:

     The `gserver' and `pserver' connection methods all accept optional
  method options, specified as part of the METHOD string, like so:

       :METHOD[;OPTION=ARG...]:

     Currently, the only two valid connection options are `proxy', which
  takes a hostname as an argument, and `proxyport', which takes a port
  number as an argument.  These options can be used to connect via an HTTP
  tunnel style web proxy.  For example, to connect pserver via a web proxy
  at www.myproxy.net and port 8000, you would use a method of:

       :pserver;proxy=www.myproxy.net;proxyport=8000:

     *NOTE: The rest of the connection string is required to connect to
  the server as noted in the upcoming sections on password authentication,
  gserver and kserver.  The example above would only modify the METHOD
  portion of the repository name.*

     PROXY must be supplied to connect to a CVS server via a proxy
  server, but PROXYPORT will default to port 8080 if not supplied.
  PROXYPORT may also be set via the CVS_PROXY_PORT environment variable.

Signed-off-by: Iñaki Arenaza <iarenuno@eteo.mondragon.edu>
Signed-off-by: Junio C Hamano <junkio@cox.net>