Add new netaccessor: curl

I added a new netaccessor to the Xerces 3.0-unstable branch.

The libcurl-based netaccessor relies on libcurl (http:// 
curl.haxx.se/), which can be built on most platforms, and is already  
present on some too. Curl supports http, https, ftp, file, ... and  
others.
My hope is that the curl based netaccessor will supplant libwww  
completely (libwww is no longer being maintained or enhanced) and  
that will it will be used in preference to the various sockets-based  
netaccessors as well.
The configure script knows to look in /usr and /usr/local for curl.  
If you've got it installed elsewhere you can configure --with-curl=/ 
my/path/to/curl to tell it where to look.
Let me know if you've got questions.

James.