Fix for Macports 2.2.0 upgrade problems (if /opt/local is a symlink)

I was having trouble updating MacPorts on one of our OSX servers.

Error: org.macports.extract for port gnupg returned: command execution failed
Please see the log file for port gnupg for details:
Error: Unable to upgrade port: 1
To report a bug, follow the instructions in the guide:

It turns out that version 2.2.0 doesn’t like symlinks to /opt/local that didn’t bother earlier versions.  See here.  The solution was to edit /opt/local/etc/macports/macports.conf in order to provide the full path everywhere.