Improved proxy support

[Bryan Blackburn]

FYI for those not following the bug [1], I've attached a patch [2] to  
ticket #13158 which improves MacPorts' proxy support from just env  
variables to several useful settings.  One reason this is needed is  
that 10.5's sudoers now cleans most env variables before running stuff  
as root.

Basically, the default with the patch is that if proxy information  
(for HTTP, HTTPS, FTP, and rsync) is set in the environment, that will  
be used.  Otherwise, if it's set in macports.conf, use that.  Finally,  
if still not found, it'll query SystemConfiguration (aka, what's set  
through System Preferences).  Note that rsync isn't settable through  
SysPref.

A setting (proxy_override_env), when set to yes, will cause MP to  
ignore what's in the environment and use either what's in  
macports.conf or SystemConfiguration.

Once a value is decided with the above procedure, the appropriate env  
variables are set in port's environment for rsync and libcurl.

Bryan

[1] - <http://trac.macports.org/ticket/13158>
[2] - <http://trac.macports.org/attachment/ticket/13158/mp_proxy.diff>