MacPorts should warn when not using "macports" rsync module

[ryandesign (Ryan Carsten Schmidt)]

MacPorts currently prints this warning:

Warning: MacPorts is configured to use an unsigned source for the ports tree. Please edit sources.conf and change 'rsync://rsync.macports.org/release/ports/' to 'rsync://rsync.macports.org/release/tarballs/ports.tar'.

This advice is not optimal: the user should be advised to change it to "rsync:// rsync.macports.org/macports/release/tarballs/ports.tar".

In addition to warning when not using a tarball, as was already done in [bebd94ad0415b859b37a9887dfae102b45e70590/macports-base], we should warn when not using the "macports" rsync module. (Using the old standalone "release" and "trunk" modules is deprecated; our new rsync server provider would like for us to wean users off of those old modules so that they can eventually remove them.)

We currently issue this warning for all *rsync.macports.org hostnames, but not all of our mirrors use the same rsync module naming convention which complicates things. Ideally we would ask our mirror providers to change their setup to match ours. But until that happens, maybe we can only issue the rsync module name warning when using the official rsync.macports.org server and not when using a mirror.