[MacPorts] howto/PatchLocal modified

[Jeremy Lavergne]

> We should not be suggesting that users put ports in /usr/local/ports. Although doing so won't cause any problems, installing libraries in /usr/local/lib or headers in /usr/local/include does, and therefore our standing policy is to not support users installing files in /usr/local, and in the course of troubleshooting we often advise users to rename or remove /usr/local. Almost any parent directory other than /usr/local would be a better choice.

Could just stick to /opt: either a new parent directory (/opt/mine) or a child directory (/opt/local/mine).